Kids.
On first glance, on a day to day basis, the relationship between you and your child is this: They take and you give. And they take some more. And guess what? You give some more. To any rational person, it seems like a crazy relationship to enter into.
But I have found (I have a 2 year old boy) that when I take some time to step back and view the forest through the trees, kids end up giving their parents far more than you could ever imagine, only they do it in indescribable ways, very discreetly changing you into a better person and providing you with a much better view of life and a more appreciative attitude for all the little things that go right every single day.
It's a relationship only a parent can understand, and after experiencing it, it's a relationship that any rational person would be crazy not to enter into (given the right circumstances, obviously).
Congratulations, man!
(This coming from someone who never thought he'd ever have children, either)