It’s usually easier to notice bad behavior in others than it is in ourselves.
You and your friend, while studying together, have to have a sandwich at three in the morning. Sweet, savory calories stuffed in a bun. They’re delicious, alright, but the late-night snack induces other bad behavior. The energy boost you get prevents you from going to sleep, resulting in an array of undesirable secondary effects.
Relatively easy to identify among the two of you. Now what?
When two people show the same type of behavior that they wish to get rid of, noticing it is a great start. External input is still required to improve upon the situation.
Two wrongs might (not) make a right, but they surely can indicate what’s wrong.