Well, I'll almost certainly remain single for the rest of my life, so it's a purely academic question, but:
In favor of Yes:
-Kids can be fun (though also exhausting). I like their 'raw,' blunt, untamed nature - yes, this means they can be very cruel (bullying, etc) but also are more genuine and can also be generous.
-Wanting, for lack of a better word, a positive legacy - not in terms of passing on my name (I think that is shallow and stupid) or my genes, or in terms of a successor, but in terms of, that if I raise the kid well, it'll be a tiny contribution towards making the world a better place, something worth doing that will outlast me. Other 'legacies' I can think of for myself seem petty or trivial.
-Like the idea of having that kind of bond with someone, less conditional than friendship, romantic love, etc usually are - it may sound selfish, but I do understand it goes both ways, that I'd be willing to always be there for them.
In favor of No:
-I don't like babies. I don't hate them, but even at their best they just seem boring. I don't think I'd have the stamina and patience to deal with a baby.
-I'm not sure of my suitability as a father at all. Sure, there are worse parents out there, but still.
-The teenage years worry me. I don't dislike teenagers, on the contrary, but I do worry that I'd have no idea how to handle one or relate, how to give them healthy breathing space and independence while still supervising adequately, etc.
-The world is overcrowded as it is, and I don't think my genes are so special as to be particularly worthy of being passed on.
-While having a kid similar to me in some things would make it easier to relate, do things together, etc, I also worry that he/she would have my problems with lack of social intuition, tendency to depression, etc.
Some of the 'no' reasons might make it sound adoption would be something to think about, but the chances of a weird single male adopting in my jurisdiction, unless he's very rich, are exactly zero - even a regular, young married couple trying to adopt a disabled kid can have a very hard time doing so (sadly not due to a shortage of kids without adequate homes but due to an excess of petty bureaucrats and stupid laws).
I strongly believe that the choice of those who don't want kids should be respected and accepted as legitimate.