Swiping pens from work seems small. The person you become doing it isn’t.

The pens are not the issue, the practice is. Every time you take something small you rehearse an argument: it does not count, nobody is hurt, everyone does it. That argument does not stay attached to pens. It waits around for something larger and turns up with the case already made.

Watch for the sentence that starts with it is only. Those two words are doing a lot of work, and they are worth hearing when they show up. Putting the pen back is trivial, which is exactly why it is easy to actually do.