WARNING... this is probably gonna be one of the darkest theories you guys will see on this.

Title--- Max and Katie

Question--- Why is Max so attached to Katie?

Answer--- He sees Katie as a mother figure.

Evidence--- His behavior.

Theory--- As a young puppy, Max either lost his mother because she died or she abandoned him. (Honestly, I'm leaning more towards the dead part but still.) When he's adopted by Katie, he sees her as a motherly figure. This is evident through some of the things he does, specifically sitting by the door waiting for her to return. But he also runs to her when he's frightened. I think Max even has nightmares at times but he calms down when Katie pets him.

Unanswered Question--- What happened to Max's mother?