If you look at Christmas movies, there are certain things in them that lend themselves to a 'Harold & Kumar' movie. In particular, the more out-of-this-world things like Santa Claus and flying reindeer.
A lot of times in movies, especially in sequels, the characters become caricatures and just sort of improv machines and joke machines, rather than people you can actually connect to.
When you hit 30, it's that time of self-reflection. Some people are a success. Some people feel like they haven't achieved what they wanted to. Some people are married, some have kids, some are still single.
When you watch the first 'American Pie' movie, you're like... 'I know that jock who's kind of sweet and has a girlfriend, and I know that weird off-beat kid.'