It really is indescribable and not for the faint of heart or the selfish. The beautiful thing about parenting is that it shows you who you really are (not who you think you are), and gives you chances every day to grow.
It makes you see what really matters in life, assuming you actually come to this realization. I've seen plenty of people not realize this and fight to keep their identity, their original idea of what they wanted for themselves while also trying to be a parent. That doesn't work.
Part of parenting is a certain amount of ego destruction. You have to go through that if you want to genuinely care for another human being.
This is what makes the experience of parenting so great. It is a kind of Zen experience of making yourself better by destroying your concept of self (and putting another 'self' first more than your own self would like).