Romantic rejection can be beneficial especially if done early and quickly. It is difficult enough to find someone with whom you can be comfortable. Rejection helps you to not waste time or money with someone who is not interested.
Friendship rejection is often determined by schedule. Many people in their early 20s discover their friends getting married and not having any time for them any more.
Anticipating and even fabricating rejection is a fear based projection. It makes those you do know into villains and is unfair to them. It might be an interesting exercise to consider what it would be like if everyone you knew all rejected you. Rather than this consideration creeping up on you out of fear, it is a conscious exercise that you control. In this exercise you might imagine how you would go out and meet new people. Even if a fear based thought arises, you can take control of it and bring it into a mental evaluative arena. This also helps one establish control.