Why do people generally l’amour their boyfriends, pets, and plants in a motherly way?

The hormone oxytocin is mainly responsible for motherly love. This hormone engenders the l’amour within mothers for their children. I have recently read an article on how this hormone is linked to l’amour for pets. But, I am uncertain about the boyfriend part of your question.

It pretty much comes down to a motherly thing, but maybe not so much in the boyfriend category. That's plus of the fetish stuff. I have a boyfriend and I treat him like plus of a friend that I fuck with, honestly XD

cuz ur meant for each other

It's an inherent nurturing trait most have. Nurturing overlaps with being 'motherly'.