I guess that makes sense. When people say things like "I want a lot of money", "I want to live in a fulfilling relationship", "I want to climb mt. everest", the essential quality of these desires is that they are real and actually happen roughly the same way you picture it in your mind. No one says things like "I want to have the good feeling of living in a fulfilling relationship whether or not I actually live in one"... no. Because it's important that they're actually real. You can say the same thing about helping others - if you don't want other people to suffer, then it's important that they actually don't suffer.

That wouldn't be a very good reason to ignore the person I'll become. After all, I do want the person I'm going to be to be happy. How is this different, in any important way that changes the reasoning above, from noting that I do in fact care about other people in their own right?

It's a bit different. You will eventually become the person you are in the future, but it's impossible to never get inside the mind of someone else, at least not directly.

people would be happy to change themselves to get warm good feels both when others are happy and when others are sad.

How would you actually change yourself? It's very difficult in practice.

People also wouldn't care about people too far away to cause changes for them.

But people don't care about far away people so much as they care about people that are similar to you. When westerners get in trouble in developing countries, people make a big effort to get them safe and mostly ignore all suffering that is going on around that. People send less money to people in developing countries than say, war veterans or people at home.

That's a fact about how I am. Why should I try to change that?

You shouldn't. I'm the same way, I try to help people for the sake of helping them. But there are some people who are only interested in their own well-being and I'm just thinking how I could argue with them.