because we are human and that's just how it is don't get me wrong people can be nice if they choose to. though some people are only nice when they want to gain something and then there are the few people who are genuinely kind :) Monet
This requires an in depth study. but in short it started at the very begging of time when man went against God and obeyed Satan. Therefore turning his own dominion over to Satan. This brought a curse over all mankind. The curse is that we are all born with a sinful nature. Depending on a number of things determines how one manifest sin in his sinful nature. That's why even children at a young age already fight over stuff and are to some degree selfish. That is also why Jesus Christ came to die. To deliver us from the power of sin and Satan. He paid the price for your and all mankind sins. His righteousness is only applied to those who put there faith in Christ and his finished work. Read Romans ch 5 vs 12-21.