Though there's no indication that Dr. Craven is a truly evil character, suggesting that Colin should spend all of his time alone and not speak to Mary is a way for Dr. Craven to keep Colin from getting any better. Readers should also remember that Dr. Craven is next in line after Colin to inherit the manor, which may be why he’s so eager for Colin to waste away. On another note, when Mary and Colin have so much fun talking about the moor and Dickon specifically, it suggests that talking about nature can have almost as much of a healing effect as actually being out on it.