Adults need to learn how to handle rejection. It's so essential to learning how to be happy in the real world.
People who are lonely because they fear rejection need to fix the fear of rejection part before they will actually experience a real friendship. Being open with each other is essential to friendship, and being open risks rejection.
Friendship isn't a individualistic pursuit, it involves multiple personalities and is full of risk and rejection. No reward without risk, contrary to what you might be sold on to mask your social anxiety.