Sometimes, as much as you don't want to feel that way, the little quirks about someone that you once found amusing are now annoying. You feel you've changed over the years, but they're still obsessing over the same petty gossip and drama from 10 years ago.
It's not necessarily about being in different stages of life, although this can be a factor. I firmly believe friendships can survive changes in circumstances, but it requires understanding and willingness from both parties.
A lot of it's about maturity. Some people grow up, some don't.