I definitely have a better workout, especially on legs, if I push myself. I'll dread walking in the gym and knocking out what I have planned out, but if the weight is too heavy or I can't get the reps I wanted, maybe even if I do get the reps I'll decide in that set I want two more....if I push myself, I and succeed I think it's a "success breeds success" kind of thing. And that helps me have a much better workout, physically and mentally.