]The town of Silent Hill is a psychological reflection of one's feelings of guilt. In James' case he saw monsters and someone who looked just like his dead wife... Laura doesn't have any hidden guilt or secrets, she's too young. From her perspective, the town is just an empty place where Mary might be waiting for her, hence she doesn't see any monsters.
Throughout the game, Laura provides information about Mary to James, and most of the time James is disbelieving. James claims his wife died years ago, whereas Laura provides evidence to suggest contrary. Ultimately, it is through Laura that we undertstand what James had done. She represents the truth to counter James' self-deception.
If you notice, Angela only ever sees one monster - the wierd thing she drops a TV on. Other than that she sees flames all the time. Eddie sees some monsters too, he thinks they are people laughing at him.
It's all very complicated!