I think there are two types of people in this world: those who cry while reading books and those who don't (Of course, there are people who don't read books at all, but let's not talk about them now. That's another story). I fall into the first category. I don't cry at every single book I read, but if it's sad, or full of any emotion really, I'll break out the tissues.
I don't particularly like watching sad movies. I'll watch them, and I'll like them more often than not, but I don't usually pick a movie to watch by asking, "Which one will make me bawl?" I don't even listen to sad music a lot (it does happen, though). However, if I see a book about a girl whose dad is sick and she has to move away from all her friends and she gets cyber-bullied and her dog also dies, I'll probably read it.
(I'm going to try to keep spoilers out of this post, but be warned, they might sneak in. Spoilers are like that, you know.)
I cry for all sorts of reasons. The last book I cried as I finished was Second Chance Summer by Morgan Matson. The thing is, I knew it would be sad from the first page! The main character, Taylor, and her family go to the lake house for one last summer together because her dad is terminally ill. I knew what I was getting into. I commented aloud on certain parts of the book: "Oh, he's talking about his favorite movie. I bet they'll watch it later." Then, fifty pages later, "They're watching the movie." But I really liked the book. I think I was joking about it because I knew it would make me cry, and jokes would distance me emotionally from it. It didn't work. I cried like a... Not even like a baby, because babies at least look cute crying (sometimes). I cried really, really hard.
While compiling that list, I realized that the tears I shed for some of these books were because they were actually sad, but a surprising number of them were books I cried at because they were my favorite children's books, and I cried for... childhood? I don't know the reasoning behind it, but it's the same reason I cry while watching the Samantha: An American Girl Holiday movie, when Samantha's Uncle Gard says he'll adopt the girls ("JENNY SAYS YES, SIR"). The movie (and the books) are so happy, but so emotionally charged, and I cry because I remember seeing it (and reading them) when I was young. I think I cry for a mix of those reasons: I cry for grief and I cry for memories.
And this is why I love books. Just one paragraph can make me cry--"Charles. Charles, I love you. My baby brother who always takes care of me. Charles Wallace, come away from IT, come back, come home. I love you, Charles." (I'm not ashamed to admit that I almost cried while typing that out.) If just that much can make someone cry, that says something about the book. Books have this amazing power over their readers, so even if you know how the book will end from the very first line, you'll still get invested in the characters. You'll still end up crying, because you went through something with the characters. If you let yourself, you'll be a part of the book. The character's emotions will be your emotions.
That is why I love reading.