Not an easy thing to put into words. I expect it's both a unique and a universal feeling among people who feel children are beyond worth it.
For me, children are the epitome of all that I love. They give and need compassion, empathy, love, understanding.
They highlight all of a parent's strengths and weaknesses, therefore you are challenged to grow and change with your kids.
They are hungry... hungry for knowledge, danger, wisdom, adventure, stories, etc and they want to be filled.
They are filled with wonder at the simple things. They have such a way of reminding us of innocence lost... bringing us back to an appreciation of the things that are truly important.
They have a unique perspective, one that we lose as adults and it's good to be reminded of. A child's innate ability to accept others as they are, naturally nurture someone who's hurting, and express themselves without fear of embarrassment is an inspiration.
As a mother, there's also just no way to explain the bond that comes from a life growing inside of you, feeling that baby move, and then giving birth to and nursing him or her.... it's just a bond that no-one can ever recreate or take away.
You pour your life into your kids. It is the most difficult, frustrating, challenging, and sometimes painful thing you'll ever do. BUT, when your child says "I love you" or "thank you, Mom" or when you see your children struggling, yet succeeding, or contributing to society in a positive way, it's this overwhelming feeling that yes... you've done something very good. But even when they don't do those things, there is just a basic instinctual love for this person who is a part of you or who you've invested so much time and part of who you are into. Because whether they are your blood or they are adopted, children are a part of you.