Very nicely pin-pointed in regards to self-discipline and control. It is everything of these two that encompasses a person to limit his/her feelings in a controlled environment. It gives you a type of confidence, something which you have also mentioned, Arshad:
Furthermore, as FrozenFire mentioned, the more stressed a person’s mind and body is, the more enjoyable and determined a workout it is. Compare your stressful days of working out to the days when you work-out with less stress (and with a peaceful state of mind), it is a world of difference in how much energy you release just by doing an extra set or knocking out another 15 minutes at the treadmill.