This is also crucial in elevators where it's especially important to size up the reflexes of your fellow travelers in case there is any kind of emergency. You also don't want to know when it's too late that someone has intimacy issues when you're curled in a ball crying and needing to be held when the elevator stops and the lights go out. You need that friendly elevator "Hello" that communicates "I've got your back, buddy. Go ahead. Be weak".
Of course, being socially open like this can lead to awkward situations as well and not everybody is particularly cool with the intimacy of even a generic exchange. Sometimes I find myself becoming angry or resentful with someone for not returning my smile or thanking me when I open the door for them. I know I shouldn't but I do. Usually I can just brush it off but sometimes I take the slight personally and offer a mumbled "You're welcome, your majesty" or "Hello to you too, I'm sorry I'm so fucking rude". They usually can't hear me but it makes me feel better. It gives me closure and closure is important.
There is also the crazy awkwardness of the overly exuberant greeting of a stranger who looks vaguely familiar and so you give the "hey you....long time no see" surprised smile and hand gesture only to discover they are talking on their iPhone headset and you just happen to be standing in front of the ham. Don't panic. That was their faux pas, not yours. First of all, if holding a 4 inch phone to their ear is too much work they shouldn't be out in public anyway. Secondly, they're a mess and everyone knows it. Just keep walking, head held high and share an exasperated eye roll with anyone who may have witnessed the exchange. Now you're bonding again.
Of course there are times when it's best to keep to ourselves and keep eye contact to a minimum.... like visiting a prison or entering a furniture store. Most of the time, however, it's nice to have a friendly connection with people whose paths we cross on our journey through life. And you really don't know if that smile you offer a stranger might be the only smile they see that day. So smile, say "Hello", open the door for someone. It doesn't take that much effort and the occasional moment of compassionate connection is worth a dozen or so blank stares. But please. No winking. Nobody needs to see that mess.