There are two types of people in the world. Although they have equal degrees of health and wealth and the other comforts of life, one becomes happy, the other becomes miserable. This arises from the different ways in which they consider things, persons, and events, and the resulting effects upon their minds. The people who are to be happy fix their attention on the conveniences of things, the pleasant parts of conversations, the well-prepared dishes, the goodness of the wines, the fine weather. They enjoy all the cheerful things. Those who are to be unhappy think and speak only of the contrary things. Therefore, they are continually discontented. By their remarks, they sour the pleasures of society, offend many. people, and make themselves disagreeable everywhere. If this turn of mind were founded in nature, such unhappy persons would be the more to be pitied. The tendency to criticize and be disgusted is perhaps taken up originally by imitation. It grows into a habit, unknown to its possessors. The habit may be strong, but it may be cured when those who have it are convinced of its bad effects on their interests and tastes. I hope this little warning may be of service to them, and help them change this habit. Although in fact it is chiefly an act of imagination, it has serious consequences in life, since it brings on deep sorrow and bad luck. Those people offend many others, nobody loves them, and no one treats them with more than the most common politeness and respect, and scarcely that. This frequently puts them in bad temper and draws them into arguments. If they aim at obtaining some advantage in rank or fortune, no body wishes them success. Nor will anyone stir a step or speak a word to favor their hopes. If they bring on themselves public disapproval, no one will defend or excuse them, and many will join to criticize their misconduct. These people should change this bad habit and condescend (俯就) to be pleased with what is pleasing, without worrying needlessly about themselves and others. If they do not, it will be good for others to avoid any contact with them. Otherwise, it can be disagreeable and sometimes very inconvenient, especially when one becomes mixed up in their quarrels. The main idea of this passage is ______.