Among the many things the aspirant needs to cultivate, there are few that are as important as cheerfulness, enthusiasm, and equipoise. When the mind is gloomy, depressed, or disturbed, its actions are chaotic and binding. Hence arises the  supreme need to maintain cheerfulness,  enthusiasm, and equipoise under all circumstances. All these are rendered impossible unless the aspirant succeeds in eliminating worry from his life. Worry is a necessary result of attachment to the past or to the anticipated future, and it always persists in some form or other until the mind is completely detached from everything.
-Discourses 7th Ed. p358