Kids force you into making decisions you never thought you'd have to make, and give you a different set of priorities. The ideas you have about yourself and about what your life is going to look like change after you have kids, and as your kids get older. In things as simple as getting rid of your expensive super-awesome coffee table, to things as complicated as deciding to end relationships that don't nourish you or allow you to be the best parent you can be. From making decisions about the kind of music you listen to while your kids are around, to deciding to push harder into your career or pull back from your career.
It's a double-consciousness. The joy and connection of parenting, but being forced into decisions you didn't know you'd have to make. Even when you're absolutely sure of and satisfied with the decisions, you still wouldn't have been forced to make them in such a deliberate manner if not for your child.
Even if it's for the best, it's still the end of your own innocence.