He arches an eyebrow at Sam for the first, implying heavily that if Sam's going to make it a dare, Steve's going to pick it up. Smart people can walk away from dares. It's not a kind of intelligence Steve has. Has ever had. Is likely to ever have.

The rest... Good news: Steve wouldn't recognize flirting if it walked up and kissed him square on the lips. The bad: He still gets a little uneasy in response to the question, because he's aware of his own feelings and that he's maybe exposing them here. He overcompensates, and looks Sam square in the eye. "Why do you think I'm so comfortable being here? Smells like you."