Willravel

Three things:
1) Millions of years of social development. Evolutionary or biological altruism developed because humans are a social species and cooperation was absolutely necessary for the survival of the species. It took more than one cave-man to take down a mammoth or to fight off a saber-tooth tiger. Because our numbers were so important, it was necessary for survival to help other members of your tribe or roving band to also survive.
2) The social contract. A part of living in society means that everyone has necessary social rules that we observe in order to maintain order. If you hurt someone or steal from someone, you breach the social rules and face real consequences.
3) "Be the change you want to see in the world" A principle I discovered that really appealed to me was a Mahatma Gandhi quote. Assuming my own standards are acceptable, modeling those standards to other people spreads a better example of the social contract.
Also, it makes me happy to be "good".
There are common moral rules for most human civilizations. Do unto others, don't murder, don't steal, don't lie, etc., are all fairly common parts of human social contracts.
Innately, it's the morality and ethics that are built in, but consciously it's pleasurable. I like helping people and being good.