And this only happens to me usually on one of two occasions:
1) When I have just acquired a new ladyfriend, suddenly I become irresistible to the opposite sex for 2-3 weeks. When I met my girlfriend I swear I had women coming onto / propositioning / (and in one case) almost raping me for a (very) brief period of time. It was nice, and I abstained each time, but it was nice to feel loved. This admittedly last occurred 6 years ago, but I guess when you're happy you give out some kind of "HANDS OFF ALAS LADIES, YOU CAN'T HAVE ME THIS TIME SO THAT'LL TEACH YOU FOR IGNORING ME FOR A DECADE" pheromone.
2) The other time this sometimes occurs seems to be when I'm hungover, scruffy, listening to uplifting tunes (today it was Cut Copy fact fans) and not really caring about my general appearance because I'm recounting what a good night out I had on my morning commute. Like today. I went to watch Kong last night (who were superb), met some very nice DiSsers and woke up this morning knackered and feeling a bit shady. But I swear I had 3 really, REALLY hot women check me out this morning. And the usual stroll to work NEVER involves this. I think the first and most 'European' looking one may have been smiling at me more because I had ketchup & bacon partially smeared over my face, but I had the confidence / lack of ability to think much about the sheer oddness of this event and held eye contact with her for a bit whilst smiling back in a suave manner.
And it was nice. This should happen more often I thought. She seemed to enjoy it passingly, as did I. In all honesty I doubt we'd have lasted long term. She was a bit vegetarian looking to me, and I reckon she'd have potentially liked Bon Iver a bit too much, but hey...
So if you're an attractive individual, I think you ought to smile at people more often. It is a nice way to start the day.