Before children (BC), it's all about you, who you are, what you want, what you need, how you feel etc etc
After children (AC), it's all about DC, what they want, what they need, what will make them happy etc etc
It's not a choice you consciously make, it simply happens usually. If it doesn't happen naturally (think of the threads where DH/DP refuses to grow up) then you'll have issues adjusting.
You are relegated and so is your partner, to last place, for the next 20 odd years (and maybe for life if you're going to help with grandchildren etc).
Yet everyone I've ever known still says they've never regretted it ! :) (Unless you ask them on a particularly bad day when it's school holidays, partner is away and washing machine is spewing water and bubbles all over the kitchen floor....:( )
But the good days far outweigh the bad ones, and the things you thought were SO important BC, often end up on the scrap heap. Not because you can no longer do them, just because you have totally different priorities AC, and no longer want to. (unless talking about getting to use the toilet alone and undisturbed......... I'm STILL waiting for that one to be accomplished and my DC are late teens and early 20's ! LOL)