You speak of ‘wet dreams’ when a guy ejaculates while he is asleep. It’s also called a ‘nocturnal emission.’
A nightly expulsion is a biologic event, like nocturnal erections. The guy may have dreamt about sex, but that’s often not the case.
There is more of a chance of having a wet dream when a guy has been excited during the day without having an orgasm. In that case the guy will probably have erotic dreams.
About 20% of the guys has the first ejaculation at night. When you don’t know this may happen it may frighten you if it does, not knowing what’s going on. Many boys erroneously think they have wet their bed during the night. But it’s nothing to feel bad about: it’s completely natural.

I’m 15, ejaculate but never had a wet dream. How do I get one?