If my eyes meet someone else's I often smile at them, I smile because I think why not, life is hard enough on all of us. What I enjoy most about smiling at strangers is that when they look suprised it makes me want to laugh really hard but I feel I have to control the laughter in this case and not just burst out so I just smile more widely and look away and chuckle to myself quietly once the urgency of the laugh eases. Some people actually scowl back and when they do I usually can't conceal the laugh with a smile, I just laugh outright. When they smile back at me it makes me feel good. It's a win-win, I don't think I have ever smiled at anyone and not walked away smiling myself either because they smiled back or because I was laughing at their reaction.
I think I have run into people who were doing the same thing to me that I do to other people, see I never expect anyone to smile at me first so it's always a surprise.