Know any "jokes to live by"?

Lots of humor is based on negativity towards others or a childish glee in transgressing the boundaries of good taste, but some jokes seem to express a kind of wisdom that helps us makes sense of our actions or the situations we find ourselves in, effectively functioning like humorous proverbs.

For example, when a friend of mine in college was juggling two girlfriends and found to his chagrin that this wasn’t nearly as fun as the average young male might suppose, I told him this joke:
Patient to doctor: Doctor, it hurts when I do this.
Doctor to patient: Don’t do that.
Another example is the one about the drunk looking for his keys under a street lamp late at night. A cop stops to help him. After a lengthy search, the cop asks the drunk if he is sure he lost them in that spot. “Oh no,” says the drunk, “I lost them in that alley over there.” So the cop asks him why on earth he is looking for his keys under the lamp. The drunk replies, “Because the light is so much better here.”

Can you share any others?