The idea that everyone's opinion is subjective and as valid or invalid as everyone else's. Thus, anyone holding any particularly strong opinions is being ever so slightly silly and amusing. But you really shouldn't take this, or anything for that matter, seriously. It just all a bit tongue in cheek. So, relax guys.

What happens when you take postmodernism a bit too far. The basic tenet of postmodernism is that we need to be aware that everything we do is subjective, no matter how much we strive for objectivity. We are always making assumptions, and we need to examine these assumptions and their origins in order to more fully understand whatever it is we're thinking about. If you take it too far, you end up with something like, "if everything is colored by our assumptions and we have no way of being truly objective, then how can we ever find the truth? What's the point of studying anything if we can never really know anything for sure about it?" This is a pretty useless attitude. A dash of postmodernism here and there is good, because it forces us to look at where we're coming from. Too much, and that's all you'll taste. Kind of like pepper.

What extreme postmodernism usually misses is that if everything we live is opinion and that there is no objective truth, then individuals have the ability to create useful and valuable truths for themselves.

In other words, if you're making it all up anyway, you might as well make up something good.

liberal irony - pursuing life as if it had ultimate meaning while knowing that it ultimately doesn't

While postmodernity makes sense in theory, actually living one's life seems to require at least a bit of "working belief" in the meaningfulness of our actions on some objective score - or a cheerful willingness to ignore the contradiction between intellectually supporting the idea that truth is relative and created, while in point of fact pursuing goals that presuppose truth.

Why does postmodern irony exist?

Possibilities:
1) We are stuck between paradigms, and postmodern irony is an artifact of the transition. It occurs when people try to embrace postmodern philosophy while at the same time "just get on" with their modern life. This sense of duality will fall away as people truly let go of their modern grasping at final meaning and ultimate purpose and embrace the contingency of human experience.

2) Irony is an intrinsic part of living within the postmodern framework. Humans just can't handle living without meaning. (Although nothing is really intrinsic in postmodernity. And look, I'm making all sorts of claims about the nature of humanity - I sound positively modern. Huh, how ironic.)