Thor lives up to his reputation in the myth of Thor and the Jotun Geirrod. If you want to know what his reputation is, check out Shmoop's files on him.
He's a fighter, not a thinker. Loki basically says, "Hey Thor, this giant's house is pretty awesome. Why don't you visit it without your weapons and armor?" And Thor goes, "Okay!" Now does that seem smart to you? Especially since Thor's been tricked by giants many, many times before? But luckily, being a bit dim is no problem when you've got colossal strength. Even without his hammer, Thor's a force to be reckoned with: The fatal crushing of Geirrod's daughters, and the general slaughter that follows, proves that.
He's decisive and quick to act. Thor's words to Loki when he realizes that Gjálp's big pee is the cause of the waves that are swamping them are "You've got to stem [dam up] a river at its source." That pretty much sums up Thor's approach to problem-solving: See problem. Find source of problem. Throw rock (or hammer) at problem. Problem solved.
He really, really hates giants. And you can understand why, given how Geirrod treats him in this story, first entertaining him in the goat house, then throwing a flaming iron at him. But even though it's really only Geirrod and his daughters that treat Thor badly, by the end of the story, he's killed all the giants in sight. That's just how he rolls.