1. Marley helped Grogan to be a human because he stimulated Grogan's sensitivity, making him feel a strong affection for Marley, just as Marley felt for him. Sensitivity is something inherent to the human being and many times, we need a very strong moment to realize that we have it. Grogan had these strong moments with Marley, who was always on his side, like a faithful friend.
2. Marley was characterized as a terrible dog, but that was what made Marley great. Marley was smart, peculiar and unique. All of these characteristics were very well represented, making everyone want it.
