I think it would be obsession, because it was not ambitious. He was treating him kindly it wouldn't be nature versus nature. He wasn't doing it just for show so it couldn't be appearances, so it would have to be obsession.
The creature has admired the dynamics of Mr. De Lacey's family for a long time. He was always snooping. At some point, the monster found the courage to meet Mr. De Lacey and he was treated with kindness by the wise old man, who was also blind and could not see what the creature looked like. De Lacey's family could on the other hand and shunned the creature, assuming that his appearance also reflected who he was in the inside.
