You probably already know that eye contact is a pretty important component of interacting with other people. Scientists recently tested this by measuring people's brain activity as they responded to pictures of angry people and pictures of people who look afraid. It turns out that eye contact plays an important role when people are evaluating potential threats.
If we measured a person's brain activity when they felt threatened by someone's glare, we'd see that the part of their brain that regulates emotions and detects potential threats is hard at work. On the other hand, if the scowling person had been glaring at something else, this wouldn't have bothered them nearly as much. And the exact opposite is true for fear. If your friend were staring at something and looking really frightened, your brain would work harder to figure out if whatever is scaring them may also pose a danger to you.
Eye contact means different things in different situations. If you're mad and looking at someone, you might be a threat. But if you're scared and looking at someone, it's unlikely that you're going to attack. So what the study suggests is that when you're studying emotions, you have to take eye contact into consideration. It also suggests that in ambiguous situations, your brain has to do more work to process other people's facial expressions in order to be able to evaluate the situation and respond appropriately.