Why am i having the same dreams over again years later?

Okay You know how you have those dreams that you always forget but don't? Like they're at the tip of your tongue but cant remember them or they're in the back of your mind? Well i've recently been having dreams that i KNOW i've had before and i get hit with deja vu majorly when i wake up, and sometimes even when i'm having the dream! One time i actually had like a sequal or continuation dream of one a had YEARS ago and had remembered the first one clearly. Right now i cant remember them but its kinda like i need a trigger or something to remember them. Anyway why would i be having these dreams over again years later? Its kinda cool but freaky too. What's going on?