What are the dark sides of human nature, especially the relationship between men and women?

My friend Mr. B, with moderate conditions in all aspects, talked about a school sister who is two years younger than him at school. I've never seen a real person, and the photos seem pretty good. They have established a boyfriend-girlfriend relationship and work after graduation. My school sister is still at school. Mr. b is very kind to that school sister and has an income after work. My school sister suddenly asked Mr. B to manage the salary card for her, giving an example of her best friend. B jun accepted. In the month given, my salary was brushed by my school sister in three days and I bought new clothes and shoes. After that, almost every month's salary became my sister's pocket money. Mr. b advises you not to spend so much money. Everyone will get married in the future, so we might as well save it for later. My sister promised, but she didn't do it. Fortunately, Mr. B's family conditions are good, there are other extra income from his work, and the small vault is enough to eat. The relationship between the two lasted for two years, and my sister graduated and talked about marriage. Mr B took his school sister home to meet her parents, and Mr B's mother gave her a Yu Pei as a gift. My school sister thought Yu Pei was worthless, and she was once disgusted. Then my sister entered a medium-sized company. She was on a business trip with her boss, and one day happened to be Mr. B's birthday. Mr. B telephoned his school sister, who said that she had checked into a hotel in a certain city. On the phone, Mr. B felt that his sister was lacking in speech and was impatient, so he ended hastily and hung up the phone. Mr. B thought it was wrong, so he contacted a local friend in a city, which is next door. Name your school sister, boss and hotel, and let me check. Through my relationship, I found that the name of the boss was indeed registered in this hotel that night, but no sister's name was registered. In other words, there are three possibilities. First, my sister has never stayed in this hotel. Second, my sister checked in under another name (this is very unlikely), and third, my sister and the boss shared a room (it is normal to register only one name when opening a room, and the hotel management was not strict many years ago. )。 I'll tell Mr. B. Later, Mr. B asked him about it after his school sister returned, and had a big fight. My sister accused Mr. B of invading her privacy, arguing that she actually stayed in another small business hotel near that hotel, because the boss could only stay in a five-star hotel and the staff could only stay in a cheap one. Mr. B really believes, really believes ... but I don't. As a friend, I helped my brother expose it and save him from pain. I looked it up. What a coincidence! There is indeed a cheap chain brand business hotel near the five-star hotel where my sister claims to live. However, that business hotel was closed for renovation during that time! The business hotel of this brand was the only branch in the city at that time, and nothing else. Of course, there are several more now, but in that year, only one really opened. B jun and his sister broke up.