The spiritual law of physics.
When someone asks for help, are others obligated to assist? In the world of Karma, meaning actions creating reactions, I would certainly say that you are your brother’s keeper especially when asked. With Karma, you might find yourself in a situation where you need to ask for help too just to let you know the experience if you don’t assist.
In the same manner, if someone says hello to you, you are obligated to acknowledge the greeting. In some manner you must make a choice. Every action causes and equal and opposite reaction. That’s the law of physics and comes from the spiritual world too. “Hello” is really a request for attention. When you recognize that call and say hello back, you’ve given a very personal spiritual gift and in the giving have received one too.
So, the next time you’re in a grocery store and someone says “hello.” Give them more than they asked for. Give them a sincere “hello” back and really listen when you ask how they are doing. Watch the expression on their faces but be prepared to go the next mile and listen.