You might say, "Oh well it's just common courtesy", but is it really? I mean if a stranger waved to you from across the street you wouldn't feel compelled to wave back to them. In fact you'd probably think they were weird. So why is it that in Destiny when we come across a stranger (exo or otherwise) in the wilderness that there is an expectation for them to return the gesture if you wave at them?
[spoiler]But still, if you don't wave back at me on patrol I will hunt you down and kill your dog.[/spoiler]
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I would wave a t a stranger, nod, smile, or say hi to a stranger who did the same. In Destiny it's situational. A guy bowed to me when playing rift, afyer I had 9 carries and 3 scores with the spark. He was standing on the spark spawn, and somehow I just kinda knew that when he bowed it meant he wanted to try carrying the spark, so instead of being a hog and trying to get it instead of him, I waved back, he waved, grabbed the spark and scored. In Destiny the emotes are a different kind of communication than in real life.