People telling you to think of others first are usually trying to suppress your individuality and the deviations from their script. They're urging you to fulfill a particular role, the role they think according to The Script that you should fill. And this frustrates them. For example, you're supposed to settle down, thus taking on responsibility for thinking about a spouse and their well-being, or you're supposed to have kids and give up yourself for their sake. Sometimes they are so adamant about The Script because it's the route in their life they took, and they seriously begrudge anyone who is not currently equally devoted to spouse/kids/ailing parents/whatever else. It's the version of the elder's "In MY day...!" that your contemporary can roll out.

But at the same time, these same people want excuses to break from the restrictions of The Script, because face it, The Script sucks. They want to go spend time making themselves happy, but it's not in The Script. So they use you as a way to give themselves permission. "You need to take care of YOU!" is opening themselves up an opportunity to say "Well, if THEY get to look after themselves, then so do I!". But of course, don't go doing this without their blessing; if you do that, you're totally flaunting it and need to be put in your place, see paragraph 1.

Social rules are complicated.