Sometimes, people walk along oblivious to what's going on around them. Whatever they have brewing in their minds prevents them from seeing and hearing the people who pass them by. However, most of the time, probably for reasons of safety, no matter what people contemplate, they still notice their surroundings.
Let's think through two different scenarios:
First, somebody is thinking happy thoughts. The person has had a pretty good day so far and walks along blissing out. If I say "Hello" or "Hi" to that person, he/she/they can add one more reason to feel good to that list of feel-good moments that make today great. The person feels noticed and acknowledged a little more than before. Not a bad day!
Second, somebody is walking along thinking about how his/her/their life stinks. Nothing's going right, nobody cares, and well, why bother. If I say "hello" to that person, he/she/they might think, "One person just noticed me." I get that one person's notice might not feel like a lot, but it's something, and something's a start. It's infinitely more than nothing. It's the beam of light that shows that a shut door can open, that there's a way through, another side.
What acts of kindness, big and small, do you always carry out?