I understand it's very ingrained in our culture at this point that this is a thing people do. But, if I decontextualize enough mentally, it starts to feel quite strange: manipulating one's brain into having a negative emotional reaction.
You’re not being well understood here, but I do this too, and have done — with everything — all my life.
The answer is ultimately that if you deconstruct and logically analyse any particular human activity it either ruins the fun and/or makes you realise how primitive and dark most forms of entertainment are. People like being emotional, for whatever reason.
I like the explanation that says it’s about learning, though. Learning somehow feels intrinsically good.
It's not negative to be moved to tears. And it's not manipulative to watch a tv show. I don't think you're decontextualizing as much as you're trying to adopt some alien contrarian viewpoint.
It might help to compare it with going to the gym or for a run. In some sense this produces an immediate negative reaction: our muscles get sore and stiff, and we get out of breath. But we still do it perhaps because we feel better afterwards, or because it helps our long term physical health.
Likewise, deliberately experiencing sad (or otherwise) emotional states has both short term and long term positive outcomes. In the short term we feel a sense of catharsis, and perhaps reassured that our feelings are relatable. In the long term we feel more in touch and less overwhelmed by our emotions.
I understand it's very ingrained in our culture at this point that this is a thing people do. But, if I decontextualize enough mentally, it starts to feel quite strange: manipulating one's brain into having a negative emotional reaction.