People fail because they lack self discipline or willpower.
Self discipline, training and control of oneself and one's conduct, usually for personal improvement
Self control, the ability of a person to exert his/her will over the inhibitions of their body or self
Willpower is a finite resource (Finite resources are those which are only found in limited amounts). Think of willpower as a muscle. Think of your bodies skeletal and cardiac muscles and how they can be trained. Like those muscles, willpower can be trained to become stronger, to last longer, go further, and be more powerful. Can you think of a time when you were in better cardiovascular condition or stronger physically? The reason you were stronger is only because you conditioned yourself to be that way. Now, can you think back at a time when you were more disciplined with work or your social life than you are now? Yes? Likely, at that time you had more willpower in many aspects of you life at that time. You had trained yourself mentally to be strong at that time. You can workout and get stronger, if you stop the workouts, you become deconditioned. You can demonstrate tremendous willpower by training it, if you stop, you lose it and your willpower and self discipline becomes weak.