Why do I cry when I think of GOOD memories of my girlfriend and I?

I'm 15 and so is my girlfriend and we've been dating for 14 months now (excluding a month or so we broke up but we still add that time into how long we've dated). Anyways, we have like around 200 pictures so far from when we started dating and when I look at them for a certain amount of time I actually tear up and eventually even start crying pretty hard. This usually takes place at night. Like I don't know how to explain this but in a way, it's kind of sad tears.. it's really bothering me because I don't know why I'm doing this. Don't get me started on these slideshows we made. For our year anniversary from when we started dating, she made me a TWELVE MINUTE slideshow of nearly ALL of our pictures and added the songs: Amazed by Lonestar (the FIRST song we slow danced to at a birthday party about 10 days after we started dating), Only You can Love Me This Way by Keith Urban and she had All My Life by K-Ci and Jojo. All My Life is the song that gets me the most for some reason. But anyway, when I watch her slideshow, I end up crying really hard and getting my sleeve wet because I cry into my arm when I watch it if it gets bad. Don't get me wrong, I love her very much and our relationship is AMAZING right now, but I get really sad when I think of our memories. I mean even memories where I look back at it and think to myself: "Wow that was literally one of the best days me and her have ever had." Even TWO WEEKS ago during spring break when I went to the beach with her on a Friday and stayed with her until Saturday. I cry about it and I don't know why. I mean it wasn't that long ago and it was a very happy time. But I mostly cry hard about stuff that happened a long time ago. I really don't know why I cry about all of this because they are very happy memories, but when I cry, I don't feel happy.. I feel depressed. Please help, it's very confusing and it makes me miss her so bad when I cry about these things.