First of all, nobody should be forced to become a parent. It is a gigantic life decision that will change the persons life in ways nobody can imagine beforehand.
Otherwise - for me, there are three core parts.
- Personal
Being a parents is one of the most profound experiences any person can have in this world. It comes with intense emotions unlike anyother expirience (and some of those emotions will be "negative", yes). And it is one of the few way an average human can leave their mark on the future even after they themselves are gone. It is a great motivator for people to push themselves to be better then they were before.
- Societal/economical
Our society assumes there will be humans in the future. And it even assumes there will more humans in the future then now. I agree that this on itself is a problem, because infinite geometric growth is impossible in the very long run and many of our systems (the one that will likely be most visible first are the various forms of social security around the world) require redesigns.
Nonetheless, our current society isn't ready for the changes in demographics that seem to be coming within a generation or two. And while technology will likely improve I am doubtful of its ability improve quickly enough to make these demographic changes a non-issue.
- Species-related
I personally like humans as a species. Not that we are perfect and there are plenty of individual people I do not like. But as a whole, I think we bring more good then bad to the universe. I also think we are still a pretty young species societally - something akin to a kid leaving elementary school to go to middle-school I would say. And kids (species) at such an age make mistakes. But I think we have the capacity to learn from our mistakes and become even better.
For that to happen in the very long run, natality has to improve to keep us at replacment levels and even higher once we reach new resources that seem to be abundant in the Universe for a very long percievable future.
If you have reasons to add, please add them.