Dumbledore acted nice, but Tom was used to people lying to him, he can tell the way faces show different emotions, it's all in the eyes, and the twitch of the lips. He is not the savior that Dumbledore was hoping to train. He thinks there is something wrong with being able to talk to snakes, which are arguably the only company that Tom has had for a long time. At least snakes while being sneaky are readable and understandable. They are in it for the food, warm basking spot, and perhaps a safe place to sit for a while. Dumbledore is not like that. He wants Tom to be a fighter. He wants him to be brave, and carry the Wizarding World as a symbol. Tom knows that is not the only motive that the other has, but he can not be sure what else the old man wants from him. But if Tom promises to train, then he doesn’t have to go back to his grandparents, he would do anything not to go back.