The short answer is, I don't know. But creating a blog isn't about giving the short answer to anything, so I'll go on.
I find romance novels, movies, and tv shows addictive. I can get totally immersed in the story in seconds, more eager to find out what happens–and by "what happens," I mean "how they end up together– than to eat or sleep or talk to other humans. I don't really understand the urgency, as I never confuse reality with fantasy and I have a totally fulfilling life of my own. Still, somehow, I need the two main characters to find each other. I can read romance books about straight characters, gay ones, trans, adults, teens, the elderly. Somehow, none of those details limit my desire to see them come together.
So, this doesn't explain why I cry, so I'll keep thinking. I want to talk about my favorite movie, The Princess Bride. (I love the book even more, but it doesn't make me cry, so I will stick to the movie.) At the end of the movie, the grandson asks the grandfather if maybe he could come back to read the story again the next day, and the grandfather responds, "As you wish." Gets me every time. (Of course, if you've never seen the movie or read the book, you don't know why that's such a touching line, but if you've never seen or read The Princess Bride, shame on you. Stop reading this post and go watch or read it!)
I haven't answered anything yet. I know that. The feeling at that moment isn't satisfaction, though I love the way the line harkens back to the beginning of the story. The emotion isn't mere happiness because seeing his grandfather again isn't the happiest moment of the tale. But the sweetness of it is off the charts. I guess I like the sweetness of it all.
What makes you cry? Feel free to write an answer in the comments.