Why do some boys who are interested in you send him messages, and he will return if he wants to, but he won't if he doesn't want to?

Several possibilities. One is that you think too much, people are not interested in you, and it is purely polite to reply to your message. Don't come back when you are free. Second, he is really busy. When he is busy, he won't have time to reply to anyone's message. Third, he is a top-notch scum. I'll give you hope first, and then I'll hang you like this to better satisfy his abnormal psychology.