Are you sure? Go and stare for several minutes right into the eyes of the biggest, toughest-looking stranger you can find, and see what happens. Eye contact is always an effort to communicate something, and it can easily be interpreted as a challenge.

Sorry, what I say was probably not clear enough. I agree with you, eye contact is a way to communicate something. It may be defiance (that's what I meant by "it might be linked"), but it could also be seduction, fear, friendly etc. And we can accept most looks much longer than animals.

I just had a strange experience in Paris subway : a complete stranger on the platform (seemed tough) and I on the train did catch each other looks for a good few seconds and ended up smiling at each other. It was not the first time that I put myself in a friendly mood and got friendly looks back at me. Pretty unusual, as shown in this (french) cartoon http://www.dailymotion.com/video/x7fp4s_18-baston-de-regard_... (you don't need to understand French to get it)

The first rule of working with non-human primates: don't make eye contact. The second rule of working with non-human primates: don't make eye contact. The third rule: take a guess. Etc.