It feels strange when someone you don't know smiles at you. However, I will say my experience: When I open the main door in a public building, I hold it to anyone behind me as a sign of respect, a sign of acknowledgment. Some say thank you, some don't. But the worst part is that when the person behind you (usually a young lady ) actually opens the other half of the door while you are holding the door for them. I don't know why they do that. Yet I still hold the door to anyone behind me. I do it because I feel it's the right thing to do. I do it because when I'm the one behind, and the person in front of me doesn't show any attempt to hold the door, I feel I'm not being acknowledged. Maybe it's just me who pay attention to this minor thing, but I cannot ignore it. The point is that you do what you feel is right, and don't expect any return to it. You do it for yourself.