Because it's the right thing to do. I've heard it argued that all parents become monsters, i've heard idiocy along the lines of it's cruelty to bring a child into the world. Well, i want to have children. I want to see aspects of myself and my wife reflected in them. I want to be the best part of their lives.
Just because people have fucked up before doesn't mean that the only way is to fuck up. Not everyone is twisted by their parents. Not everyone had a crappy childhood. Some people come from families that i envy beyond belief but i wouldn't swap mine, however other people may judge them.
The logic behind wanting to have children is actually pretty simple. It's based on love. I'm a married man and i love my wife more than i could ever tell you - or her. And i want more! I want more of her to love, i want to love those aspects in different ways, i don't really feel that i'll ever be complete without expressing all of that in every way that there is to express it.
I'll never see the moments of happiness, the childhood fears, the formative experiences that made this person that i love so much but i can get to see those of her children, our children. They would be what we most had in common, our greatest and proudest achievement however they turned out and whatever they became. They'd be the multiplier of everything that's good in our lives.
And not having kids because of some silly notion that everyone gets it wrong is frankly absurd. You might as well say "I'm not ever going to try to do anything differently from anyone else because everyone else does it that way". Well, i'm not a sheep. I'm not bound by their restrictions. I am the latest and greatest version that the human race has produced and i'm evolved far enough to fucking well get it right. And my wife is the best person i ever knew. We can do better than our parents because they made us to be better than them.
And our kids will be better than us. That's why. That's all i have to say.