New alt, or new game?

Some people only create alts when they have a specific purpose in mind. Maybe you really want a healer, perhaps there’s someone you really want to play with and levelling alts together is the easiest way, or maybe there’s some tradeskill you’d like to have (in your stable of alts) and you need to level an alt to learn it.

But sometimes it’s just to take a break, or do something different. eg. I’m a bit bored, I’ll go level an alt!

So how do you decide whether to roll a new alt, or whether to just go  play a different game?