Tamaki thinks over this explanation, and he thinks he gets it. Steven is a really nice person; of course he wouldn't want to upset anyone by being sad, especially if he thinks they can't do anything to help.

"So you dressed like this, thinking everyone would know you're sad?"

Just going by their conversation earlier, he's going to guess that didn't go quite as Steven planned; though, Tamaki realizes he's not exactly known for being the sharpest crayon in the box. It's entirely possible that only he missed the implication of what Steven was doing.

"Do you want to talk about why?"