Graduate school is great at weeding people out. For the most part, it does what it should: it lets the independent thinkers through. Theoretically, if you are capable of making decisions, thinking critically, and you actually put the effort in, grad school should train you to use those abilities in a specific field. And then you graduate.
Practically, graduate school selects for other traits as well. One of these is the ability to overcome the fear and dread of rejection.
Let's face it. Rejection is a normal part of the academic experience. If you do a quick internet search on "academic rejection", a host of articles will appear on all aspects of rejection. Rejection in writing. Rejection in grants. How to handle rejection as a academic. Dealing with the emotional side of rejection. Dealing with the professional side of rejection. And on and on and on.
As a grad student, you don't really need to know all these nuances at first. What you do need to know is that you WILL get rejected. Often. And it will almost always have nothing to do with you.
Now that you know what's coming, here are some reasons why rejection is not your fault.