The people who are to be happy fix their attention on the convenience of things, the pleasant parts of conversation, the well-prepared dishes, the goodness of the wines, and 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 person would be the more to be critical. 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 change this habit.
Although in fact it is chiefly an act of the 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 frequency puts them in bad temper and draws them into arguments. If they aim at obtaining some advantage in rank or fortune, nobody 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 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.
According to the author, the major reason to cause the difference between happy and unhappy people is ______.

A. that they have different life styles
B. that they have fewer friends
C. that they look at things differently
D. that they are not on good terms with the people around them