People’s hearts are hard to fathom: People’s thoughts are hard to fathom.

To belittle oneself: to look down too much on oneself, describing an inferiority complex.

"People's hearts are unpredictable, so why belittle oneself" means: It is difficult to guess what others are thinking, and we cannot understand what they think. Why do we have to look down on ourselves?

ps: The simple explanation of the trial is as above, if you have any questions, you can ask~