I'm a very emotional person, and it's tough for me to hold back any form of showing emotion, especially crying. My emotions are likely to be the last thing that matures in me, because even now, it's impossible for me to fight back tears if they want to come out, even in public, which is a real hassle for someone who's trying to be an adult in an adult environment such as college, and soon, the workplace. Luckily there have been very, very few recent instances, and I'm getting a whole lot better at toughing out small things without triggering auto-tears.
I'll also get choked up and shed tears even doing something as simple as listening to music, whether cheerful, mournful, or simply powerful. I'll cry during movies when no one else does, too, often simply because of the soundtrack in the context of the movie. Pokemon movies in particular, for some strange reason, strike something in me to cry. Perhaps it's a strong nostalgic feeling they bring. Happy things bring tears to my eyes just as easily as depressing things, especially those that are nostalgic or that comfort or bring a sense of relief.