To look good. Some common exercise programmes are about nothing but burning as many calories as possible, so it's no surprise when people end those harsh exercises exhausted, angry, and unmotivated. One and done workout program training, when done right, will really make you look good. Weakness is not an indicator of a productive exercise; what matters is that you do the best and better your process, gradually, when potential. It's absolutely acceptable (and encouraged) to complete the weight lifting exercise feeling better than when you got.