A major part of ‘humor’ is that it is a “group thing”. Humor is almost always about reaffirming bonds to your “group”. With group I mean a group of people sharing your values and/or experiences (not necessarily friends, you can be member of many different groups).
I see it as a form of effective communication to deliever one’s position on a subject.
PS: 
Hmmm, I assume that you’re not saying that humor, and by extension value, is subjective - rather, that the choice of focus and emphasis depends upon relevance, which is contextual. But that is true of any work of fiction or non-fiction, not an attribute of humor per se. The best humor, just as the best fiction or non-fiction, deals with the widest abstractions that affect all men. So it isn’t really dependent on your group, is it?
Also, just because humor is an effective way to deliver a serious, rational message (that X behavior is wrong) doesn’t mean that that’s it’s only valid purpose - there is also the value, in joyful pleasure, that thinking and laughing brings. Obvious jokes don’t work - there has to be some mental processing involved to reach a sudden insight, and that intellectual playfulness, that demand for rationality, and the emotional response of laughter, gives humor value.
I agree that humor is often used to negate or deny the importance of the irrational, etc, but what about the silly things…like, say, laughing at a kitten playing with a ball of yarn. Or maybe chuckling at a baby who has mashed his face into a birthday cake? Why is that humorous?
I think it depends on how you define “humor”. Certainly those things are not worthy of ridicule, but if you want to widen the abstraction to also include whatever happens to be mildly amusing or entertaining, or just plain random, then yeah, I guess you could say those things are funny. Personally I wouldn’t, and I don’t really find those examples funny at all.
On the other hand, joy is joy at existing - so anything that affirms your existence can be a source of laughter. I suppose that if the baby or kitten are in some way an existence-affirming thing for you, then laughing as an expression of joy makes sense.