Why? Because in all areas of life whether it be in character, self-discipline, faith, job skills, relational interactions, education, etc., we are expected to continue a process of growth and maturation accordingly.
"When I was a child, I spoke as a child, I understood as a child, I thought as a child, but when I became a man, I put away childish things. " 1 Corinthians 13:11
If we become complacent in striving to develop and grow then we also have stepped into settling for a type of dormant, unproductive "self" - something less than who we are intended and created to be. We then rob ourselves of the abundant life God has chosen for us, as well as cheating those we love and others' around us out of experiencing the best of who we are and given the opportunity to draw from what we have to contribute.
To be 'who you are' doesn't mean you are to stop learning how to become "the best you", you can be.
"Not that I have already attained, or am already perfected; but I press on, that I may lay hold of that for which Christ Jesus has also laid hold of me. I do not count myself to have apprehended but one thing I do, forgetting those things which are behind and reaching forward to those things which are ahead, I press toward the goal for the prize of the upward call of God in Christ Jesus." Philippians 3: 12-14