Had two REALLY mad (in a good way ) dreams last night, really vivid and can remember em perfectly, but cant think WHY i had em (Wasnt on mind and dont usualy dream like that), or why i remember em (I never remember dreaming), anyone know anytihng about how the mind works?

2 main people involved wasnt on my mind and never are anymore really, tho on monday when we was out we saw one of em and we talked bout the other, but totaly wasnt on mind since.
Infact when i went to sleep had 2 other totally different people on my mind

So anyone know why you dream about stuff, esp when its stuff you was NOT thinking about before you went to sleep? And why you remember dreams?