Stitch didn't know how to interact with Sammy. Sammy seemed to be more interested in doing his own thing then playing with Stitch.Stitch didn't understand why Lilo and Nani would get a dog in the first place?!?
"We're they replacing me?",thought Stitch to himself. That thought almost made him sick. His stomach flipped. He couldn't stop watching the dog chew on his chew toy. Stitch sighed. He remembered how Lilo had always wanted a puppy, but they got him instead. They thought of him as a freak and they couldn't even say it to his face. Stitch had no reason to live in their house anymore. He figured running off to a family who would actually want him as the best choice.