If you notice...being the president has aged him FAR more than if he'd not been president. Just look at pictures of him in 2000, then look at him now.
Granted, he's been president for close to ten years, but still. It's very demanding job.
I may not have agreed with him on everything he said or did, but a man (or woman, if Hillary had won) who is willing to accept the responsibility of a job that demanding has my utmost respect.