FYI it's a riff on Carl Jung: "Everything that irritates us about others can lead us to an understanding of ourselves."

Agreed, it's a valuable insight.

How would this help?

Let's say that I find it annoying when people "missbehave", like people letting their dogs shit wherever and not cleaning up, or just commonly being selfish and rude.

How do I now understand myself better? Is the point that the problem is somehow in me or what does this advice mean?

- accord toltèque de miguel ruiz ("whatever happen don't take it personnaly") - projection (in psychology)

It correlates closely with another one on the list: "You are as big as the things that make you angry."