My concentration and intensity increase dramatically when I use heavier weights. I bear down and make sure everything is just right before I lift the weight. On the other hand, I've noticed that with lighter weights, my mind sometimes drifts a bit and I even lack focus. That's because I'm not being challenged, and because I'm not driven by fear of injury.
So my latest sessions featured increased weight, with fewer reps - which also helped me focus. Concentrating on 6-8 reps is easier than zeroing in for 10 or 12. Plus, if I'm lifting more weight, I can decrease the chance of injury by lowering the reps.