The type of music you listen to affects the way you perceive the world.
Listening to happy vs. sad music can affect the way you perceive the world around you.
The brain always compares the information that comes through the eyes with what it expects about the world, based on what you know. The final results in our mind is what we perceive as our reality. Therefore, happy songs that lift your spirits make you see the world around you differently then that of a sad person.