"I dare you" is a challenge to do something that is at hand. For example: You and your friends are chatting at a restaurant, one of your friends is looking at the raw food on the counter before being prepared and says: "This will be great when done!", Then you say: "Try it raw. I dare you!" It means that you are challenging your friend to eat it raw. It may also have have a sense of fight of revenge. For example: You are with your friends again, and one of them say: "Hey, your girlfriend is pretty! I think I'll dance with her!", then you say: "Are you crazy?!?! I dare you!!!" It means that if he does that you are going to put up a fight or revenge it somehow. If you say: "Don't you dare!" It means "don't even think about".