What causes us to dream what we dream and when?

I am prompted to write about this because I dreamt about an old boyfriend the other night. A person I haven't thought about in, oh, probably 20 years. Why, all of a sudden, would I dream about him? And, maybe more importantly, remember the dream? ... because I rarely remember what I've dreamt about. (I was visiting him and his wife in their kitchen .... or what my interpretation of his wife and home is.... as I don't know where he lives or if he's even married....)

A part of me wonders if he's okay. Did something happen to him or a family member? And another part of me feels guilty for even thinking about him. Which I find very strange! There's nothing to feel guilty about LOL

More questions that will never be answered.... just another strange dream....