Happy neurochemicals can’t work if they’re on all the time.
Four neurochemicals cause happiness: endorphins, dopamine, oxytocin and serotonin. Each evolved to do a different job. When you know what the job is, you know why your happy chemicals can't be on all the time.
http://www.psychologytoday.com/blog/greaseless/201105/the-brain-needs-downs-have-ups