There are often several reasons for choosing a topic: First, interest. This topic attracts you and you find it interesting. One is useful. After you have done this topic, many people, countries and so on can benefit from it. Useful means valuable, or has social or economic benefits. The third one is very challenging. Maybe you don't know this topic, but many people haven't worked it out. As for me, I want to have a try. Another answer is that I can't do other topics, but I can do this one.
Math master Chen Jingrun, someone asked him, why do you want to do math? He said, I can do nothing but math. Doing physical chemistry requires experiments. My hands are clumsy, so I have to do math. Doing research projects is just interesting and useful. This is very interesting. Let me give you an example. I did a research. The theme of the study is to have a rusty compass. What is a rusty compass? It turns out that the two legs of the compass are movable and can draw big circles and small circles. If the compass rusts, it can't move back and forth. With this compass, what can you do without support? At that time, an international question was raised: I will give you a line and use this rusty compass to find the midpoint of this line. If you can solve this problem, it belongs to junior high school students. If you can't solve it, it's a problem that doctoral students can't do. For two or three years, no one gave the correct answer. This is an interesting question. Compasses can only draw circles at both ends of the line. You can't connect it. Find the middle point. This question is very interesting and challenging. Because after listening to it, I didn't know in advance whether I could do it. Can't do it, you can't do it if you prove it to him, or you can prove it to him. Sometimes it is useful.