Young fun teams don't stay young and fun forever. They get older and expectations rise. It is fun while they are meeting those expectations but once they stop to -- things go sour. Apathy and disinterest follows. Large section of fans wanting to start over again and rebuild.
Like the Hawks teams built around Joe Johnson, Josh Smith, Al Horford, Marvin Williams.
I think this hits it mostly (though I would change meeting expectations to exceeding expectations).
It's usually always fun when you're continually exceeding expectations.
The Kings are exceeding expectations, that's fun. The Celtics have so far currently failed to meet expectations, that's not fun (or at least as fun as most thought it would be). Had everyone expected the C's to only be a 40 win team this year, a lot more people would be having fun with this team.
Like the C's fans, I can see how some Sixers fans aren't having as much fun
currently as they were last year. Last year the Sixers exceeded expectations, this year currently 5th in the East, they're falling below expectations. Most C's and Sixers fans consider their real season to start in the playoffs though, and are just biding time till then.
The Warriors are no longer continually exceeding expectations, that's when it can get boring. It's a rare occurrence to be that dominant though. Usually the problem is you can't get past the 1st/2nd/conference finals round. So when that happens, many fans want to blow it up, start over.
Bucks fans are probably having a blast as they're exceeding expectations. I'm sure they thought they'd be good, but not best record in the NBA good. If they get bounced in the 2nd round though, next year probably won't be as fun unless they exceed expectations again (70+ win pace) or make it past the 2nd round. 60 wins won't be as exciting for the Bucks next year though when they're just meeting expectations.