what are we built for? as any species, to reproduce. why do we need to reproduce? to keep ourselves reproducing.
Survival of the fittest gives you a goal to head for, the fittest survive, and produce superior offspring (hopefully superior) and as the fittest of those survive, the species evolves to a much better version. It is a loop that doesn't allow for side roads.
I am still waiting for signs we have our 'fittest' surviving and reproducing a superior version of ourselves 
Being a bit more serious, now, I see better things in my children than in me, better things in me than in my parents, and I hope for my grandchildren to be better than their parents. Each succeeding generation owes it to themselves to try to be better in some way and no worse in any way, than the generation before it.
children are a way of telling the future you are not really afraid of it, you want to be part of it.
And until humans figure out that there are many different ways of being part of the future other than through propogation, I suppose us 'with' will likely feel a bit sorry for and possibly 'superior' to those 'without'.
There is something to be said for knowing there is a chance your genes will live on thousands of years after you have died, that somewhere in the future a descendant of yours may do something absolutely world-changing.
On the other hand, you could wind up spawning the ancestor of another guy with a bad moustache. You take your chances, but as long as you have children survive you, the possibility exists. Without children, that possibility is wiped from existence and your genes are gone from the available mix forever.
I think those who do not reproduce by choice do not really need to explain themselves - unless they are an only child and their mother is looking for grandchildren. In that case, I do feel a lot of sympathy for you.
Any way, some random thoughts on the subject, trying to get a 'fix' on the most common reasons behind 'with' 'without' for those who are able to choose.
