We never know the quality of someone else's life, though we seldom resist the temptation to assume and pass judgement.

This quote is saying that we (as individuals) don’t know what’s really going on inside another person’s life. Although it happens often, it’s not right to make assumptions about someone else’s life and judge them based on those assumptions. Instead, we should be mindful of the fact that everyone has their own unique experiences and background and should be respected for that.