A separation from God and consequently from love. Selfishness is the exact opposite of love. (Have you ever heard the expression of ‘love is sacrifice’?)
The human soul instinctly yearns for God: to be completely united to its Maker and the fountain of all love. Such is the true desire of our soul. This is what we were created for and living out this vocation in its full would be living a life of “self-lessness”.
Unfortunately, many humans often do not recognize this desire in its fullness. They recognize a deep, authentic desire for something, but often cannot put their finger on what this something is. Original sin, lack of formation, etc are possible causes for this. This opens the door for many to attempt to fill this desire with self-gratification.
The farther a soul falls from God, the less likely they are to recognize the true nature of their souls longing. The farther a soul falls from God, the less likely they are to recognize thier sins; in this case, selfishness.
It’s a harsh cycle for those who have never understood the yearnings of their soul. For those who do understand these desires yet still struggle at fulfilling them, there is the Sacrament of Reconciliation and the graces therein.