This Doonesbury is more smart than it is funny, but we here at The Comics Section are a cultured sort, so we can appreciate this kind of thing.
Trudeau is talking about the tension between our desire to see people change and our desire to see them stay the same. Change is inevitable, to a degree, but at the same time, that change comes so slowly that we don't even notice unless it's been years since we've seen each other. What's more, people will never change in the way we want or expect them to. And what's still more, if they were to change in the way we wanted or expected them to, we would inevitably be disappointed.
Trudeau is also talking about all of this as it pertains to characters in serialized fiction, which is even trickier. If characters remain the same for too long, a comic strip or a television show or whatever will become stale. But if the characters change too drastically too quickly--and thus unrealistically, like the old friends at reunions--they become fake, and the show becomes a shadow of its former self.
The scale is nearly impossible to balance. But this is what we ask of our artists. We're bastards like that.