We are significant to each other. And the line is drawn at the point where you are harming another or they are harming you. The line is drawn where you decide you are more important that someone else, or decide that you are less important than someone else. That is what morality is all about. That many wrap it in gods and tribulation and eternal punishment is irrelevant. We are born, we live, we die. And it is only actually important to us.
But since all we have is ourselves, that makes it pretty darned important. Far more important than a final ego trip in a classroom with a rifle or by oneself with a chair and a rope.
If you think life is supposed to hand you something fantastic and be so cool that even friends and love and happiness and good deeds and charm and smiles and caring are nothing in comparison, then you are in for a big disappointment. But if you are willing to be a friend, to love, to be happy, to do good deeds, to charm and smile and care, you'll find out that it is pretty cool.
Then you die. But at least you have something to miss that is real. Otherwise all you will miss is what you think it should have been. And none of us have that much control over reality.