Carl Jung reminds us that "Everything that irritates us about others can lead us to an understanding of ourselves." My challenge for others and myself is that whenever there is a flaw noted in someone else (friend, family, etc.) or when you take notice of things that appear flawed in others, ask yourself why that person's flaw irritates you.
I often say that a person who has an intense dislike for a certain character trait in others, usually possesses that very same quality. The next time that you assert that you dislike or see sarcasm, pettiness, sadness or anything else in others, ask "Why does that particular flaw bother me so?" Then remove your lenses and turn them around. Take a look within.