As a father, and stepfather, I'll try to explain it the best that I can:
I have wanted to be a father since I was twenty-three, or so (I'm thirty-seven now). I hated that I was this self-absorbed person, and I desperately needed to find something that I loved more than I loved myself. I married young, to an older woman who already had two sons. We found out that she couldn't have any more children, and it tore me up. Years later, we went our separate ways, which is when I met the woman who would become the mother of my daughter.
When she was born, it was like this weight was lifted, and all of these inconsequential things that I spent years worrying about--the opinions of others, my opinion of myself--no longer mattered. As corny as it may sound, when she was born it was like she added an extra chamber to my heart. It was the greatest day of my life, and I was the first person who got to share space with her, one-on-one. It was in the nursery, and I just sat there and watched her sleep. I remember thinking that "this is the beginning of our journey through life." From that day on, I knew that if I played my cards right, I would never be alone again. The photo albums would be filled, memories would be made, I'd have this little girl to walk down the aisle, and eventually grandkids.
One thing that I hate is when parents--or anybody, really--gets this notion in their heads that parenthood should be a universal fact of life. No, absolutely not. Some people simply do not want children, or understand the point of it, and that is perfectly okay. Nothing is harder on a child than having a parent who will treat him/her like they were an unwanted mistake, a detour that they didn't want to take. For myself, the science of survival does not matter. I just felt like I was put here to be a father, and that it would be the only thing that I was truly great at.
Anyway, I hope that clears things up a bit.