I'm always reminded of this guest lecturer I had in college. He was a buddhist monk that was giving a talk about external forces and influence.
He made the point that everything outside ourselves is constantly trying to impress upon us, both passively and actively. Billboards, advertisements, the media, our friends, our family, music, our culture, the weather, time, our jobs, the color of a room etc. etc. all act upon us and attempt to make us feel or think or believe something.
Waking up to an ugly day can completely change the way your day goes. The color of a room can make you feel relaxed or tense or happy or sad.
His concluding point was that we cannot stop these forces, they simply exist, but what we can do is learn to recognize when these forces are acting upon us so that we can then choose whether or not we let them change us or how we let them change us.