Victor Frankl, from Man's Search for Meaning:
"... it can be seen that mental health is based on a certain degree of tension, the tension between what one has already achieved and what one still ought to accomplish, or the gap between what one is and what one should become. Such a tension is inherent in the human being and therefore indispensable to mental well-being. We should not, then, be hesitant about challenging man with a potential meaning for him to fulfill. It is only thus that we evoke his will to meaning from its state of latency. I consider it a dangerous misconception of mental hygiene to assume that what a man needs in the first place is equilibrium or, as it is called in biology, "homeostasis," i.e. a tensionless state. What man actually needs is not a tensionless state but rather the striving and struggling for a worthwhile goal, a freely chosen task. What he needs is not the discharge of tension at any cost but the call of a potential meaning waiting to be fulfilled by him. What man needs is not homeostasis but what I call "noo-dynamics," i.e. the existential dynamics in a polar field of tension where one pole is represented by a meaning that is to be fulfilled and the other pole by the man who has to fulfill it."