God created man with a body, a soul and a spirit.
The original intent of man (from God's perspective) was for mankind to be the vessels through whom God would bring His eternal economy into time and space into the earth. To this end, God created man to receive His Holy Spirit within himself. When man receives the Holy Spirit within himself, the spirit of man is reconstituted back into the purview of the One Holy Spirit. This "life in the Spirit" does not end because the Spirit of God is eternal. God's motivation for this is love.
This is the baseline standard and expectation for all men was this: that they receive the Holy Spirit.
The first Adam, however, brought sin into the whole race of mankind. By Adam's sin, the body and soul were elevated above the spirit that made Adam (and all mankind) "living beings".
Therefore...
Man is permitted to resist the Holy Spirit (God) and live by the means of his own will and desires. This is the life of the soul that is informed by the body. So, instead of being informed by the Spirit of God for direction and life, man may remain beholden to fulfill the needs of the body by means the man may decide for himself. This is "life in the flesh". Because the flesh dies, life in the flesh also ends. Life in the flesh is not eternal.
If a man does not resist God by exerting his soul he can then receive God by the spirit that is in himself. The reason there is no place for the man to brag about this occurrence is because the thing in the man that can receive God's Holy Spirit was given to the man by God.
This is the gist of the matter.