Ugh, I'm horrible with eye contact. I spend 90% of my conversations staring absolutely ANYWHERE but the person's face. It's just horribly unnerving looking into a person's eyes. The only time it doesn't bother me is when I'm all rage-ified and angreh.
I used to never look people in the eye but I have a habit of always doing it when dealing with people. I'll look away every now and again but that's mostly because my eyes get tired of looking at one thing for too long.
At work: -Assume it is because they want something. Either to ask a question or purchase something.
On the street: Stare. Until they look away or say something so amazingly asinine that I laugh at them and keep on my way.
At the bars: -If woman: Strike up a conversation. Or just smile and nod their way. If a man: -Smile at them and ask them why the hell they are looking at me. Then get ready for a brawl.
If ladyfriend: -Assume that it is the signal for them to make me a sandwich.