All forms of tumah stem from the chait of Aitz Hadaa in that they possess the characteristic of death. Adam was punished with the decree of dying and the separation of his soul from the body. The severe tumah of contacting a corpse requires a taharah from the ashes of the Parah Adumah. Tumas Sheretz and Neveilah is also only if they are dead and had a loss of life. Tumahs Keri comes from the death of potential life that the sperm had just lost. Tumas Niddah is the death of the unfertilized egg in the ovaries.
Yet to say it more succinctly, it is the loss of life not death that causes tumah and the need for Taharah. Let us explain this with a concept from the Pachad Yitzchak. There is a mitzvah of והלכת בדרכיו to emulate Hashem's ways. This requirement is derived from the fact that we are created in the צלם אלקים the image of Hashem. One can demand this "likeness" from a creation only if it possesses an intrinsic similarity to each other. Only from Man can Hashem expect the fulfillment of such a seemingly impossible mission because of his neshamah that resides inside his body.
All these varying middos originate from a super middah called מקור החיים the Source of Life. Hashem is the Kadmon of existence better known as the First Cause. This middah precedes all the others because they are only expressions of life energy that was created by their creator. If so Man is required to duplicate as much as it is humanly possible this middah of מקור חיים to give and to foster life to his surroundings. We find this yesod in a passuk (Yeshaya 59,15) להחיות רוח שפלים להחיות לב נדכאים that Hashem occupies Himself with giving life to the low spirited and revival to the hearts of the downtrodden. Hashem's primary middah which is manifested to Man is not only the giving of life energy and existence but also "supporting" and "repairing" the broken spirits and hearts of those who have lost contact with the Source of all Life. See Rambam Hilchos Megilah 2,17
מוטב לאדם להרבות במתנות אביונים מלהרבות בסעודתו ובשלוח מנות לרעיו שאין שם שמחה גדולה ומפוארה אלא לשמח לב עניים ויתומים ואלמנות וגרים שהמשמח לב האמללים האלו דומה לשכינה שנאמר (ישעיהו נ"ז) להחיות רוח שפלים ולהחיות לב נדכאים
Greeting a person who you realize is in a bad mood, by a smiling good morning, is an example of להחיות . Hashem does this to us every morning by bringing us the warm radiant smile of the sun. Had this phenomenon not happened we too would all be in bad moods that day.
ואתם הדבקים בה' אלקיכם חיים כולכם היום When one cleaves to Hashem by duplicating His ways he automatically contacts the source of all חיים. The greater the similarity the greater is one's connection to life. Tumah is associtated with the loss of life and consequently with the distancing from Hashem. Taharah is the is the connection to Hashem who is the source life of every existence.
אתה מחיה את כולם (Nechemya 9,6) . מקוה ישראל ה' Yirmiya 17,13 Hashem is the mikveh of Yisrael in which Yisrael attains purity from spiritual impurity. The Bnei Yissaschor explains esoterically how a mikveh removes tumah from a person. There are four elements in the world, fire, wind, earth, and water. Each one possesses the other three as secondary elements. Altogether there are sixteen elements. Chazal say concerning the Halachos of Bittul that if the permissible food is 60 times greater than the prohibited food which got mixed into it, a nullification takes place and the soup becomes now permitted. This is the Halacha of .בטל בששים However if a Berya which is an entire bug intact fell inside a kosher mixture then it cannot be nullified even with sixty times of heter since there is a special significance given to an entire creature. However, the Yerushalmi paskins that even a Berya can become battul if exists 960 parts of heter against the one part of issur. The Bnei Yissoschor explains the rationale behind this number as follows. There are really sixteen elements as we explained above. Each element is battul by 60 parts of heter. 60 times 16 equals 960.
A human being is like a Berya. A Mikveh contains 40 seah. Each seah of water measures equal to 24 Lugin thereby making the minimum shiur of a kosher mikveh to be 960 Lugin. Therefore when an individual immerses in the Mikveh with a state of tumah, even as a Beraya he leaves tahor. מה המקוה מטהר את הטמאים אף הקב"ה מטהר את ישראל . Everything of creation contains some combination of these four elements which correspond to the Four Letters of יהו'. Man's avodah in this world is to nullify them and the middos that they represent to the will of Hashem. In order to do so one must bring to his consciousness the reality of אתה מחיה את כולם that you Hashem give life and sustain all of these elements and therefore we must transform them into vehicles to service the חי העולמים . When he possesses this consciousness it is tantamount as if he is toveiling in Hashem מקוה ישראל ה' this is hinted in the gematriah אתה מחיה את כולם which equals the shiur of a mikveh 960 lugin.
To remove the tumah that came about being in contact with a corpse, one must have the ashes of the Parah Aduma and Mayim Chayim sprinkled upon him. What is meant by mayim chayim? The water must come from its own natural source such as from a flowing spring. Water from a pit or pail that was filled by man is passul. Here we see that tumah is related to death and Taharah to chayim and life. Hashem is alone in the ability change a tamei to a tahor. The Medrash goes further to say that only Hashem can bring about an Avraham Avinu the first Yid from a father Terach a worhipper and spreader of idolatry. It is only when one connects to the Source of all Life can one alter his essence no matter how evil it is.
The mitzvah of Para Adumah is contradictory to one's intelligence. The person upon whom the ashes are sprinkled becomes tahar but the sprinkler himself becomes tamei. Things become unintelligent when we remove ourselves from the source and attempt to understand them using our peanut limited brain. If one would have emunah which is beyond intelligence he would find peace with it because emunah takes one to the source of life and at the source he will discover that everything is really only Hashem Echad.
Some individuals will respond to all of the above with a sigh and say Easier said than done! Arriving at the destination of the אתה מחיה את כולם" consciousness" and holding on to it more than five seconds, is incredibly challenging for most people even while they are davening, let alone at other mundane times. Baruch Hashem Another part of our parsha offers us an indirect route in order to attain the same goal. When the plague of snakes against Am Yisrael commenced, Moshe created a copper snake which was miraculously hanging in mid air ושים אותו על נס . Anyone who was bitten, if he looked at the copper snake he would be cured. Rashi writes that it wasn't the copper snake that brought the refuah but rather by looking at the snake they would remember that they acted like the Nachash Hakadmoni whose speech was used to make Chavah rebel against Hashem. Once they realized their shortcoming they then turned upwards directly to Hashem in order to subjugate their hearts to their Father in heaven and rededicated themselves to Him.
We see from this a great lesson. Why couldn't Moshe direct them that as soon as they are bitten they should immediately turn heavenwards to do teshuvah and be cured? The answer is sometimes one needs physical objects that will inspire him to get closer to Hashem. Mitzvos such as tzitzis, succah, tefillin are all physical which help bring about consciousness of אתה מחיה את כולם . Some individuals need a Shalosh Seudos where they sing deveikus nigunim in order to cause the stirring of the heart to feel desire to come closer to Hashem.
The Rebbe Rev Zusha alludes to this alternative method in his interpretation of the passuk (Ovadaya 1,18) בית יעקב לאש בית יוסף להבה ובית עשו לקש. Am Yisrael possesses the natural fire that creates deveikus with Hashem. However that fire must be first ignited. It needs fuel. Sometimes even the straw of Aisav the mundane and the physical can be used as the fuel to ignite the expression of one's desired love towards Hashem. I still remember the warm soothing lullaby niggun that my mother a"h would sing to put me to sleep. If I sing it now I will suddenly be overcome with choked up emotion even sometimes to the point of tears. It is at that very moment that I would channel my longing for my mother towards Hashem and start feeling the deep yearning to come closer to Him. I discovered that I was no longer crying for my mother even though my tears did not stop falling. They now had a different meaning and purpose. They were my מים תחתונים, tears of deveikus and tefillah to bond and become one with my Creator.