From Bechira: How Free Will Works by Dr. David Lieberman
Without perspective, all of the good in our lives remains out of focus. We are left in a state of restless desire, goaded on by a sense of entitlement, with an inescapable finale: disappointment and despair. By comparison, a healthy perspective advances an attitude of appreciation and gratitude, which itself changes the quality of our lives.
A summary of research:
Feelings such as happiness are a direct reflection of our choices
The following is research out of Harvard University:
(See book for footnote references)