Revealing mistake: At the beginning of the movie, when Melvin catches Simon's dog, he holds it away from him, with his arms stretched. When the dog begins to pee, you can see that the urine is coming out from Melvin's glove. (00:01:30)
Revealing mistake: Midway through the film, Carol is on a bus heading to Melvin's apartment in Manhattan. She is going to tell him that she won't sleep with him after he has given her son free medical care. She is in a pensive mood and the camera pulls out showing her in the window. As the camera pulls out further, the readable sign on the side of the bus says "not in service." (01:02:55)
Answer: It has everything to do with his mental illness. His OCD condition has kept him isolated with anti-social tendencies. He just finds it difficult to directly interact with people.
raywest