Amanda Stays at Church

What is self control?
The restraint of action toward a particular desire {A}.

But why?
Because that desire is in conflict with a stronger love {B}, and acting on desire A would hinder the fulfillment of the more highly valued B.
Let's say one desires to eat nothing but cookies, and also desires to be relatively healthy. If the desire to be healthy is greater, one will be able to refrain from subsisting entirely on cookies.

St. Augustine defines sin, therefore, as disordered love. If one loves/desires anything more than God, one will naturally place other things above pleasing God and following his commands when there is a conflict of desires.

ref: Tim Keller