It's really the Serra fight. He was just much more aggressive earlier in his career. Afterwards, he's cautious. He could have finished several of those fights that went to decision, but ever since the Serra fight, he's been afraid of that out-of-the-blue-punch that can lose him the fight.
It's definitely not that he wants to inflict more punishment or pad his stats. If I recall, for example, in the Condit fight, he was quite aggressive until Condit hurt him in a striking exchange, then GSP went back to his safer fighting strategies.
I think you're just not going to see GSP finish fighters anymore unless they are no threat to knock him out on the feet, and not enough of a threat off the back that he can go all out in gnp instead of just doing enough to maintain position. In other words, I don't think you'll see GSP finishing anymore fighters unless he completely outclasses them.