The sad fact is that people didn't need to decide why to have kids. Children came as a natural result of living. There were few reliable methods of contraception. Preventing children is against the teaching of Catholicism (yes, yes--they've okayed the rhythm method, but I'll bet someone could easily argue that having sex knowing you couldn't conceive is right up there with withdrawal). Fertile women could do nothing more than raise kids. Many families may have considered a child 'just another mouth to feed.' Children were a common commodity and not necessarily well-received.
I'm fortunate to live at a time and in a political environment when reliable contraception is available. My reproduction is almost completely within my control without sacrificing an essential part of what it means to be human and in love (sex--hopefully good). I am glad that I've been able to decide to become a mother.
By most accounts kids are difficult, inconvenient, costly, time-consuming, and resource-consuming. When they're infants, they wake you up, need to be fed frequently and you have to change their diapers. When they're toddlers they have nightmares, have a hard time going to sleep, still need to be fed frequently, and the diaper situation may be less frequent--but the job is necessarily bigger. As they become kids there's education, toys, play dates, and toilet training. All along you are constantly watching out for personal hygiene, removing trash more frequently, doing extra laundry, cooking, shopping, putting away toys, asking them to put away toys, trying to teach chores. It is easy to understand why people choose to not have children.
I'm not exactly sure why I knew I was going to have kids at even a young age. Perhaps it had to do with people around me talking as though it was an inevitability. In fact, even though I knew children could be rewarding, I was completely surprised by the sheer joy I felt after having them.