i tear up whenever i think about a friend...?

So basically i have a friend that means a lot to me, mostly because she always helps me out whenever i need it. She was valedictorian, (so obviously really smart), but she was also a genuinely nice person. we did a club together, and she was always really encouraging towards me especially, because i was new and needed a lot of help. I always think about the memories i had with her and this particular club, and i really want to thank her for all her help, and really touching my life that no person has done before. I am not a touchy-feely kind of girl, so this is weird for me, but i really want to thank her. The only problem is that she is studying at an ivy league, and I'm in high school. but back to the actual question, why do i tear up when ever i think about the memories i had with her and other member of the club? are they tears of happiness? I'm really confused....and i know this has nothing to do with it, but i also tear up when ever i sing...? what? and again, I'm not an emotional person, so i don't know what's going on! can someone please explain to me why this is happening? not that it's a bad thing, but I'm really just curious.