What makes you say that?

What makes you say that?

What has led you to that conclusion or observation? Can be used sarcastically to indicate that someone's remark was very obvious. A: "I don't believe this is going to work." B: "What makes you say that? I've run the numbers multiple times." A: "I don't think this city is very safe." B: "Huh, what makes you say that? The boarded-up windows and vandalized shops?"
See also: make, say, what