Why do we have deja vu?

I have the same thing. Sometimes its everyday, and then its months or years before it happens again. I don't understand it myself. There are alot of things we don't know about our minds. Maybe while we are sleeping our brain sometimes works in ways it usually doesn't and we are subconciously dreaming of the future and we remember bits and pieces here and there. lol, I don't know...just a theory.