Taking care of each other has noting to do with "a" god. We are all manifestations collective making up what is termed god.

Just tell them to try being a jerk and see what happens.

Relevant question if you are 12.

It can help keep you out of trouble, legal or otherwise.

Human emotions and feelings towards others aside (those lead us to do it anyway), we all live on this planet together and if we don't treat each other with respect and care the whole place turns into a nightmare. We're all on the same freeway and those who drive like idiots gain little and often get other people killed or injured. It's prudent and logical. It's also within us as humans to treat each other with respect.

If you're the only one doing it, it doesn't offer any advantage. In a hypothetical world where everyone ruthlessly exploits every advantage, no matter who it hurts in the process, someone with a social conscience is at a massive disadvantage.

When we do it collectively as a society, the reward is that other people with the same attitude treat you better. Perhaps more so if they can see you making an effort to be like them.

Of course there will always be a few who are totally self-serving. And they'll normally find their way to wealth and success off the backs of others. If wealth and status are your primary goals, there are times when you're going to need to tread on other people's toes to get there.

Because it enriches lives in the here and now. Many organisms work together as a group to achieve a common goal

But science has shown that altruistic behavior releases endorphins

Empathy, sympathy, reciprocity.

Because it makes the world a better place for us all to live in. We make people happy instead of sad or angry. Happy people are less likely to hurt us.