This is such a huge question and so widely debated because there is no one answer.

A guiding principal I use for my life is "if no one knew you did something, would you still do it?" That is, are you acting selfless for the admiration of others? Are you acting selfish because others might judge or take advantage of you? If so, it's my opinion that you're letting dictate your life. There's nothing inherently wrong with that, you get to live as you choose even if that means going along with others, but if it's causing you distress then following your own course might be a better option.

At one point in my life I tried to harden myself a bit. Focusing more on my self and less on others because I thought that was what adults were supposed to do. And while some people did admire me for personal keenness, I didn't admire myself. I was stopping myself during private acts of kindness or accommodation for fear that people would see me and judge me and condemn my supposed weakness and it hurt me every time I did it. It was an act that eliminated the appearance of weakness in exchange for the growth of an actual weakness.

It was a good experience in some ways, I learned to draw lines and how to say no. But now I do those nice things and let the few bad seeds take advantage of me, because I have the peace of mind to know that I let that happen of my own accord, instead of forcing people to take it only after cracking a cold hard shell.