To be honest I made it up on the spot, I typed in a few more things first in the place of doughnuts, but I worried they might offend someone. So it ended up being doughnuts, non-offensive little circlets of saturated fat filled joy that they are.
The term I've heard was "friends are like orgasms..." but then I remembered some people suffer with an illness that sees them randomly orgasming all the time, which they don't like so better to be safe than sorry. Nobody randomly doughnuts.