An infectious twist of hormones and biology. I've always hated kids, swore I'd never have them since I was four years old, and never felt a shred of maternal instinct. But after coming off Lexapro and the ensuing manic episode, I developed a bad case of baby fever. Can't say if it was due to being infatuated with someone at the same time or just a wonky reaction—maybe a bit of both. Interesting regardless.
I still get it occasionally, and bad, but it's purely emotional. In all rationality, I don't want to put my ills on a child, literally, and I'd be an unfit mother by definition. Yes, I would love to have my partner's child(ren), but I won't because it would go against my morals to traumatize a kid with my parenting, even an adopted one. Also, it'd further plummet my quality of life and remove suicide as a possibility, which is my current escape hatch "in case of emergency".
I admire cute kids from afar and sometimes let myself fantasize, but I keep the knowledge that it's just biologic conditioning in the end, and I must resist.