It's amazing how much we humans tend to thing about ourselves. It's something called ego or our self.
From the moment we wake up, till the time we go to bed, we are constantly thinking about ourselves. Studies suggest we concentrate on ourselves more than 95% the time. And the remaining time we concentrate on things like relationships which indirectly is again concentrating on our own selves.
That is what makes us humans selfish. If we would spend just a few minutes everyday getting interested in other people, we would know how beautiful and interesting the lives of people can be. We would find many interesting things about people. If we would just try to find out what makes a person as he/she is instead of judging a person, it would be much more beneficial for us.