"He really loves her and is scared (of) what others might do to him in a relationship," hollywoodlife.com quoted a source as saying.
"He thinks that they might use him or have different intentions. He is comfortable with Kristen, and he is happy with her, and she knows it. He feels that she is the best for him," the source added.
The couple met on the sets of movie Twilight and have been together ever since.