It's what makes us human. It's gives us Theory of Mind, the knowledge that others have minds too, you know, consciouness - everything flows from that. Altrusm is a by-product of empathy. Without empathy why would we do anything for someone else?
It's interesting to note that sociopaths can actually exhibit empathic behavior even without feeling the emotional drive to do so. Those sociopaths who tend to get press time are not
only sociopaths, but also have additional psychological issues like poor impulse control, among other things.
The reason is deceptively simple - emotional attachment to others is not a prerequisite of altruism. Human beings are a social species, and our society requires integration with other human beings in order to function, there are direct non-emotional benefits associated with altruistic behavior, including social status and recognition, the simple fact that acting to benefit the group collectively ends up benefiting the individual, and the Prisoner's Dilemma - even if one feels absolutely no guilt or shame or empathy, one can still recognize that treating others in a particular way typically leads to reciprocation. We choose what kind of world we want to live in by how we treat others, and that applies to sociopaths just as much as everyone else.
I wouldn't say that empathy is what "makes us human." There are human beings who have a cognitive defect preventing or dulling the emotional reactions we identify as empathy, and they are still people with hopes and dreams and desires and thoughts and feelings. The vast majority of them will not commit any great crime, but will appear to be perfectly normal people at first glance, and will still not actually be "evil" or "inhuman" if you spend a significant amount of time with them. Statistically speaking, it's highly likely we've all met at least a few sociopaths and had no idea; they may not even know, themselves.
Instead, I think what makes us human is the capacity to have those thoughts and hopes and dreams and desires. "I think, therefore I am," essentially.
It certainly does not seem as if there is any supernatural requirement to explain the appearance of altruistic behavior and the ubiquitous "Golden Rule." It's simply
rational, and it remains valid even absent the emotional "gut feeling" usually associated with the term.
The human understanding when it has once adopted an opinion (either as being the received opinion or as being agreeable to itself) draws all things else to support and agree with it.
- Francis Bacon
"There are two novels that can change a bookish fourteen-year old's life: The Lord of the Rings and Atlas Shrugged. One is a childish fantasy that often engenders a lifelong obsession with its unbelievable heroes, leading to an emotionally stunted, socially crippled adulthood, unable to deal with the real world. The other, of course, involves orcs." - John Rogers
A world that can be explained even with bad reasons is a familiar world. But, on the other hand, in a universe suddenly divested of illusions and lights, man feels an alien, a stranger. His exile is without remedy since he is deprived of the memory of a lost home or the hope of a promised land. This divorce between man and his life, the actor and his setting, is properly the feeling of absurdity. — Albert Camus