King Solomon in Proverbs 27:19 states the basic principle of how people react to one another: "Just as with water, the face [in the water] reflects the face [of the observer], so, too, is the heart of one person to another." Whatever expression is on your face when you look into water, that is the expression you will see staring back at you.
So, too, if you feel positive about another person, that person will feel positive towards you. But if inwardly you feel negative towards someone, even if you do not verbally say anything bad to him, he will have negative feelings towards you.
Today, greet five people as if you were totally thrilled to see them. If this is too exuberant for you (or them), then modify your approach. Try smiling and waving to three strangers. Notice their reaction.
Another idea: Think of someone you know who would appreciate an exuberant telephone call of well-wishing.
(see Vilna Gaon - Proverbs 14:17 and 27:19; Rabbi Pliskin - "Consulting the Wise")