See, This is a classic example of selfish or selfless. I have a sister who is a very selfish person. She will only do something if it benefits her. She will do stuff for others but only if she gets something from it. It's hard to be around. Like she will always get lipstick on the glasses she drinks from and it stays till washed by hand. So I asked her if she would simply rub it off before putting it in the dishwasher. Her response, "Well I don't mind it. " Me: "But I do" Her: "So? ". The thing is, Not caring about others is simply selfish. And most people HATE being around selfish people.
Should implies some obligation to put others happiness before your own. I will agree that in some cases it is good to put others happiness before your own, But you don't have an obligation to do so. Even if you enjoy being selfless, That doesn't mean you always have to be, And it doesn't mean that other people have to.
If putting everyone else above yourself is considered "selfless", Then most humans are extremely selfish. Of course, We are. If we weren't, We'd be at the end of the food chain. Consider little things like a love triangle in a movie. I could bet most of us have rooted for one of the love interests. If the main character was "caring about others' happiness more than their own", They would just date both of them since they wouldn't want to break someone's heart by saying no. However, Choosing one of them would be considered "selfish". This little incident says a lot about our society. I would also like to mention that like I said "most humans" previously, These people we call "selfless" are people with extremely high degrees of empathy (Level 7), And most of the times, These people are suicidal, Because they "care about others' happiness more than their own. "
If we aren't happy, Then how will the people around us be happy, Most people, Especially in a household are dependent on each other, So if one person is emotional, It is likely that everyone will get affected, Moreover, Bad vibes can easily spread, Easier than good vibes at that!