How do you react when someone isn't interested in the thing you're interested in?

I guess it depends on the person. But if it's me who likes something and someone reacts indifferently or negatively about it, I'd back off unless I care enough and suspect I can get them to change their mind and have some appreciation on the subject, especially if I think it would benefit them too in some ways.

But if I focus on myself, then that would be a tell of care or lack of it. Someone who is interested and who cares about you ought to show some enthusiasm, if not on the subject, then on the way that you light up about those passions. They'd want to listen to know about you and understand you and show interest just because they like to be around you when you're feeling like yourself.

But that is easier said than done admittedly. We can care for someone but admit that we don't care as much about people or things that matter a lot to them.
It's an imaginary wall and a reason to evaluate on the relationship actually. You can care, but it can be less than the amount you hope to be able to say you feel.

For the people I feel responsible for, I make time to listen. To inquire and even to bring up what I know are their passions. Why? Simple. I like seeing them happy and letting them feel heard and cared for.

If it’s someone telling me something that I’m not particularly interested in, I’ll try to be nice about and listen and might even inquire about it, just so it isn’t awkward. But more often than not, it’s me sharing something I’m interested in that no one else cares about. It’s always awkward and embarrassing for me, but at the same time, I’m used to that happening so that I don’t even bother anymore. Maybe I’m just not that interesting in general and I’m better off keeping to myself. I can at least say that I try to be nice about it with others when they get rambling and avoid making the exchange uncomfortable. That’s often more than what the other person does for me when I’m the other end of it.

I usually chat with people about whatever they are interested in - except for a few subjects like sports and mechanics - at which point I'll gently find a way to move off and chat with someone else.
If I don't agree, I'll test the waters to see if they can handle a discussion of differences and the reasons.
If they can't handle it, I'll focus on asking about how they came to that view or interest and what it means for them.

If someone is not interested in something I love, I shift the focus immediately to what interests them.

It's a common problem. I've rarely met anyone interested in as wide a range of topics as I am. Most people are happy to talk about themselves. It makes me wonder - maybe that's the norm.

ive seen this too. usually, i back off if I think the other person isn't interested at all. if they are the one interested in something and im not as interested, I usually try to be curious about what they are interested in to see if i can find common ground with them.

imo if it starts a big fight, then (idk how else to put this) one or both of the people needs to learn to think about themselves a little less and about the other person a little more.

LOL, if you ever want to stay married, you need to pretend to care.. My husband is an over the top sports fan.. I am only a bit interested... I play it up at times

It makes me feel sad when it happens. I usually shut down when it happens to me... that's not very productive.

I have learned to make the content fit the audience.
Like for example, if I were passionate about music, I would discuss those topics with people shared the same passion.

I say pfffffttttt

Try to find common ground…?

You stop talking. People have different interests. If someone gets pissed off that's an ego out of control.

Why do you think that people need to share the same interest in things that you do? Who put you in charge of what they should focus their attention on?