I'm fascinated with all the "I believe" answers, but there is a better answer based on science. Humans are a social species of animal, and social species are regarded as highly interactive with members of their same species and whose psychological well-being is associated with social interactions. These social interactions require members to have a level of cooperation and altruism. You can observe this in very young children, who notice unfairness (are "inequity averse") by the age of 3. When this is encouraged, children tend to grow up more emotionally healthy.
If inequity aversion is discouraged, however, this can lead to a later life of anti-social behaviour.
More importantly, however, humans -- unlike some other social species -- can default, or act independently of fairness concerns or altruism. A beloved dog, for instance, will often fight to the death to protect its pups or its owners. Ants and bees will die protecting the colony or hive. Humans, through the use of reason independent of instinct, have little difficulty defaulting.
When humans cooperate, we accomplish great things -- we can build entire cities. This is true for termites, building enormous, air-conditioned nests for the entire colony, or for cooperative wolves laying a trap for prey. But humans do not always cooperate. One of the reasons for this is the very flexibility of our cooperation. The social insects (bees, ants, termites) are generally very closely related. The workers in an ant colony, for example, are all the offspring of a single queen. This, of course, helps the queen's genes to get passed along to new generations, so the evolution of this sort of cooperation is easy to understand.
While humans also tend to live in family groups, and are highly cooperative within them, we have evolved the flexibility to cooperate among other, more distantly related, or even unrelated. Humans have scaled up cooperation from hyper-local, small-scale family units to large-scale societies through devising effective institutions and norms to incentivize people to cooperate when they might otherwise be tempted to cheat or to “free ride.” For example, institutionalized forms of punishment, like the police, help to coordinate a more cooperative equilibrium.
We are also unlike other species in that we’re very concerned with how we appear to others. We often behave more cooperatively when we believe that we might be observed and when our reputation is at stake. Just look at the real world, in which many contemporary foraging societies, where individuals hunt for meat that will be brought back to the camp and shared with everybody. And, surprisingly, it’s often the most generous hunters that occupy a central social position and are able to leverage social capital themselves if they ever fall into a time of need. It's easy to see the long-term benefit, isn't it?
Now, look at the human reactions to our most recent pandemic, or to the threat of climate change. Our response to the pandemic so far has been rather parochial and piecemeal, as can be seen in how vaccines have been distributed around the world. We are really interdependent, and we’re not going to get out of the pandemic until we’re all out of the pandemic. But this doesn’t seem to be the thinking of various governments around the world. Although we are a social species, and interdependent, we humans have also evolved the capacity for independent thought and action.
I rather think that this is because independent thought and action can help to pass on our own genes. As noted, ants and bees can't do that -- their genes are common throughout the colony or hive. But human social cooperation spans gene pools, which I think tends to weaken cooperation imperative. And if that goes too far, you wind up at some very selfish, and often very bad, behaviours.