Maybe you two are starting to get pissy at one another now that you have been "together" for awhile again (I mean literally together, as in not living on opposite sides of the ocean). I get along very well with my older sister when she is at college and during 1 week breaks like Thanksgiving, but over summer we eventually start to get a little more irritible with eachother more often.

That's normal though. Maybe it is a pet peeve of his. Would I get mad if a friend of mine told me that? No, I would jokingly tell him to **** off and that I don't follow standard refridgerator etiquette, cause that's how I roll with my friends.

So yeah... That's very very normal, and unless it becomes a habit and he does it a lot it's not really important