Mixed Signals

Have you ever been in a situation where you meet a guy, decide he's a nice guy but don't really want to pursue anything, but you are nice to him, and he takes that and goes gung-ho with it? You show any signs of being nice to a boy and they think you are totally into them! Why is that? Don't get me wrong it happens with girls too. For example a guy holds a girls hand during a movie and here the girl is thinking "oh my gosh! This means our relationship is moving to the next level." and the boy is maybe thinking if anything at all, "hmm her hands are soft; this movie is freaking awesome!" Why can't there be a global signal that everyone knows about? Why can't there be the three second stare rule like in the movie "He's Just Not That Into You." It makes for a very awkward situation.