Lots of different reasons, some reasonable, some less so.
Some dumb reasons would be social peer pressure, status symbol/ political beliefs, or some ppl think they get a pet, a toy or a slave out of it.
This is not talked about, but I think becoming parents for selfish reasons is actually about as common als false friends who just want your money& popularity or romantic partners just want sex, attention or bangmaid/walking ATM.
I'm pretty sure my father sperm donor just had me & my siblings cause "a good catholic is supposed to" & that makes him feel superior to others (& because he thought his wife would do all the work)
I'm certain he would've been much much happier if he never had us (& he let us feel that, too. The ppl that give others shit about not wanting kids are always the worst parents. Insufficient Justification Effect & Sunk Cost Fallacy are one hell of a drug.)
Some better reasons are some ppl enjoy taking care of others, like interacting with small children/ family activities in general, or they're extroverts & want there to be many people to interact with.
Supposedly breastfeeding a baby produces the greatest release of oxytocin the human body can produce.
Ovsly those are very subjective & are not gonna be true for everyone.
Ovsly the process of making a baby (especially as the birthing parent) & handling them especially the 1st year is very unpleasant so it would only seem worth it if you really, really enjoy babies. Ppl who enjoy babies tend to find it super rewarding.
Usually those baby crazy ppl know it from a young age tho, like they often relate that they liked to interact with siblings, cousins & neighborhood children for as long as they've known.
My mom was always jealous that she was the one kid on the block with no cousins & liked taking care of her brother as a kid & babysitting as a teen. Plus she's an extrovert so she likes it when there's noisy humans in her house. (while she didn't do everything perfect, someone who genuinely enjoys babies is a night& day difference compared to her husband who clearly had us out of peer pressure)
I'm personally not a baby person & have no parental instinct whatsoever, but they can't all be making it up.
Lots of ppl do (probably cause many of us are the descendants of ppl who liked babies)
2) It's fun / feels good.
That said for every drug there's gonna be some ppl who just don't like it or where they react to it negatively. Like that person that gets anxious from weed.
I've heard of ppl who tried meth or coke & decided they just simply didn't like how it feels. (hence, it should absolutely be respected if someone decides they don't like something especially if they've tried it & didn't enjoy it)
If you think it's not enough fun to make up for the possible health risks that's totally legit. (you're prolly not gonna have extreme adverse effects if you just use occasionally tho.)
I feel that way about nicotine. Tried it once or twice, but imho it just doesn't get you high enough to be worth the relatively big laundry list of adverse effects.
It comes don to a personal risk/reward calculation, like any risky thing, like driving a mortorcycle, doing extreme sports, going camping in the woods, having lots of casual sex etc. In the end none of us are gonna live forever so the one who "wins" is not the one who lives the longest but who has the best experiences.
Though liver failure or lung cancer are ovsly not great experiences tho, so... you gotta have some balance.