Pixar movies. It's because of a mix between happiness, sadness and nostalgia.
Finding Nemo, my favourite movie that I've seen countless times, makes my eyes tear up in the happy parts because I'm happy - when Marvin and Nemo reunites, when Marvin enjoys the maelstrom, when they survive getting swallowed by the whale etc. But the sad parts are worse, of course - when Marvin tries to follow the boat but loses sight of it I still panic, and I'm embarrassingly torn up when he sees Nemo "dead" and then abandons Dory.
"Aranka, why are you crying? You've seen this movie a million times, you know it all turns out good."
"WELL MAYBE IT DOESN'T THIS TIME WHAT IF THEY NEVER FIND EACH OTHER
"
Also, the beginning of Up made me cry like a baby... both times I've seen it. It's so touching!
Oh, and Forrest Gump. The first time I saw it I cried from the moment his mother died until the end of the movie - I was a complete wreck. The thing was I thought the movie was over soon when his mother died and allowed myself to start crying, but it's wasn't even near the end. I just couldn't stop because it was all so sad, and when it wasn't I accidentally thought of his mother or Bubba again and it all started over.