The first lesson I learned after coming here. Smile and say hello to everyone you make eye contact with. What a difference it makes to the other person and you!
If you are ever in India and find that people just look at you and look away or continue staring, don't think they are rude. May be some of them are rude or just creepy and we don't mind being rude to others. But mostly, we only say hello to people we know. Also, smile is a way of telling you are interested in/attracted to the other person. So, we don't want to get in trouble :).
Anyway, I sure did learn to smile and greet. It makes me feel good and happy. It makes me feel like I am living in a friendly environment. I like not having a straight face and looking grumpy all the time. So, smile!