I think it has a little bit to do with how babies learn to laugh. Really, if you look at how babies are held and played with by their parents in _healthy_ ordinary families, you'll see it.
Laughter happens when you see something that _might_ appear odd, threatening, awkward or disturbing....but it isn't so. It happens when you see something "fearful" but then realize it's harmless and safe.
So a sense of humor depends on there being at least _one_ of two things present, if not both of them. There needs to be a properly trained sense of courage present--cowards never laugh because fear is _always_ a real thing to them, and not to be mocked. And there needs to be some sense of _Safety_ in mind--a frame of reference for what is safe and harmless.
In a world that is perpetually _unsafe_ people end up fearing and hating everything about the world. Humor gets replaced by paranoia. In this world people only laugh _at you_. It becomes one more attack.
In a world where no one learns proper courage, everything is serious because there's no learned sense of perspective. Bunnies and butterflies become threatening because nobody told you otherwise. This happens more in cases of neglect rather than outright abuse--because it's more of a case of simply not knowing the difference (for example) between trolling for insults and a real personal attack. It can happen when someone is TOO sheltered or in TOO permissive an environment as well.
A lack of either one can warp one's sense of humor, and a lack of both? Yeah, keeps one from developing a sense of humor entirely.