Well at two levels really.
At a basic level, humans are social animals and we are interdependent on each other in social groups, initially family and then at wider levels. This has proved to be a successful method of survival. If you have I am alright jack sod you, mentality, you will not risk your life, but then no one will risk it for you. In times where we faced real physical threat, such as sabre toothed tigers and and for economies of scale you couldn't hunt, gather, protect and rear children without the help of others.
As we have progressed and become more sophisticated we have developed the notion of morality and developed a culture of what is socially sanctioned behaviour. So we look after not just ourselves but provide all sorts of institutions to care and control the populace. Therefore institutions such as public health not only provide health care for the sick (morally worthy recipients of altruistic provision), but also control the diseases that could affect the wider population.
So we attempt to stop young men killing each other because it is a terrible waste of life and the victims families suffer dreadfully. And, because if we do not control young men and curb their aggression, the potential for civil disorder is very high