Why do so many people choose duty over personal happiness?

In another question I asked recently, people seemed to universally choose duty over selfishness. There are only a couple handfuls of answers so it might change, but I doubt it.

It is fascinating to me that the good of the group (duty towards others) seems to be more important for most people. They eschew personal happiness in order to serve others—usually family members.

It’s like communism—people choosing to give what they have in order to give others what they have not. Ironic, in America, which is supposed to be motivated by selfishness.

So is personal happiness not what it’s cracked up to be? Are people afraid to be selfish due to some consequence or other? Do they feel like they don’t deserve it? Help me. What’s going on here?