In any situation, encounter, or setting, you have a choice: You can respond with positivity or you can respond with negativity. Some might say there is a third option, you can respond with indifference, but indifference is a lack of caring and, thus, is a form of negativity.
You have the choice. Your choice will influence others around you. Therefore, since positivity puts something good into the world, and negativity puts something dark into the world, make the choice to be positive. Try to find the good in the situation, or person, you're coping with. This is, perhaps, the hardest thing to do, especially for some who suffer from emotional impairment like depression or grief, but try. Make the attempt.
There will be times when you will fail, when the negativity and darkness will well to the surface almost against your wishes. That's okay. Sometimes you have no alternative but to acknowledge negativity, for it would be inappropriate to do otherwise. When a loved one dies, there an overwhelming sense of loss, and loss is seldom positive, for example.
But if you have a choice, put more positive energy out into the world. You never know when your choice will have an impact on someone else. Perhaps your nod and smile just changed someone's day. Perhaps you found a positive spin that another couldn't see and you've allowed them an alternative path to follow. Trying to stay positive, and send messages that reflect that attempt, help everyone involved, especially you.