Why am i dreaming of this girl i don't know but love?

For the past 2 1/2 years there has been this girl that has appeared in my dreams. When she is there we are either kissing and having a loving conversation or she is there is passing (our eyes always meet though). It's hard to explain but I can see her face but can't at the same time. By that I mean I feel like I know her well and love her yet I don't actually know what she looks like other than having dark hair and wearing a white or cream colored sun dress. Its like her face is wiped from my memories as i see it but she is fimiliar and beautiful. In the last dream I had abouwt her we where laying on the floor next to each other and she kept asking me not to leave (she was crying). I kept kissing her and telling her I would always love her and before I woke up we where just holding each other. Since then I havnt seen her. The thing is when I would wake up I would miss her and the dreams felt so real. Like they where legitimate memories or stretches of time. I'm a firm believer that sometimes God allows you to have dreams that are more then just regular dreams, such as with loved ones who have passed away. Who is this girl though? Why did I dream of her in that way and then why did she/I leave? BTW I have only been I one serious relationship that ended after these dreams started but it isn't about her obviosly which doesnt bother me a bit lol.