I look at this a little differently. When you have access to any track well ahead of anyone else, combined with consistently great dancefloors, of course you're going to experiment as much as you can and try anything once or twice. He does that, at least in my experience. At the same time, when a track goes off and you fall in love with it, you're going to play it again, until *you're* tired of it.
And so I think he plays songs until he gets tired of them. He clearly loves every track he plays, and knows each one inside and out - you can see when he mimes random instrument parts, vocal samples etc. The guy really listens to his records. It's different than someone who chooses what to play based on an audience's taste or expectations.
Compound this over time and you have an infinite and growing record box full of 10s...I prob wouldn't want to waste time on more than a couple 8s or 9s per set, either.
I'm as guilty as anyone for preferring the new/different/never-been-played-before records...and personally I do wish his sets had more variety. But hey, he gets up there and does whatever he wants, and that's why we love him. He's mellowed out now but for years he was pure id in a DJ booth. I've seen him play the same track twice in a row on a few occasions, and I wouldn't call it laziness...more like Sven being Sven.