I think the reason some people get happy about their post count just comes down to human nature. People, for some reason, like to belong to groups. And this forum is, in a way, a group. Post count is just a way of showing how active a particular person is within that group. It doesn't mean, of course, that those people are respected any more than a person with only 5 posts. People are judged based on the quality of their posts, not the quantity.
Getting excited about reaching certain post count numbers, of course, doesn't really make alot of sense. And neither does getting extra happy about the year "2000" or the new "millenium". It's just a number--that's it. Why do people get so much more excited about the number 2000 than the number 1992? No real reason at all. A long time ago, someone developed the numbering system. And at some point, some people decided to get extra excited about round numbers, marking them as "special". "20th anniversary" is more exciting to most people than "21st anniversary". Why? Absolutely no reason at all. People just like reaching certain (usually round) numbers. I guess it's part of human nature.
It's silly and kinda stupid, but who really cares. If someone wants to get happy about reaching a certain post count, let them. Yes, nobody but that person will really care, but as long as they're happy, who cares?