Hot and Cold BF?

Me and my boyfriend are very much in love but...i dont understand him he turns hot and cold just when i think everythings perfect he shifts attitudes.He acts like im the only thing in his world that matters cant keep his hands off me and the next he acts like im nothing towards him.And i dont think i do anything to cause it,he just i dont know it just all of a sudden happens.I want to talk to him about it but i honestly dont know to confront him about it.Whats going on with him?How do i talk to him about it?please help thank you