It's nothing genetic...

At least not for people

Capsaicin is what makes peppers hot, and all mammals are sensitive to it. But birds aren't.

And birds are better are distributing seeds than mammals, so some peppers that evolved to have a lot of capsaicin spread much further. There was an advantage to large mammals not woofing a whole pepper down in one bite.

The difference in people is some like the endorphin rush from their bodies thinking they're in actual pain, and some people don't think it's worth it.

But the vast amount of people that don't like spicy food never work up to it, they just go straight to something crazy spicy and then refuse anything remotely spicy.

Like, if your first time drinking alcohol you just chug a fifth of everclear, it's probably gonna be a while before your second night drinking.