Thousand island dressing
I like to toast the bread, melt the cheese over the beef under the broiler, and have a hard time limiting myself to just one. But then, you find that you've run out of Swiss. You still have half a pound of corned beef, most of a loaf of bread, and an almost-full jar of sauerkraut. Buy some more Swiss, so it doesn't all go to waste.
Then you run out of bread. Replace it.
Then you run out of beef. Replace it.
What's a reuben without sauerkraut? Replace it.
And on and on and on...