Personal experience (his and mine). Same basic principle explains why people work together in offices instead of at home: peer pressure prevents you from slacking. And personal trainers take this up yet another level, because it's a lot harder to give up when somebody's in your face telling you that he knows you can do one more rep.

And yes, of course there are people who can put in supremely productive 9-to-5 days at the home office and/or whip themselves mercilessly in solo workouts. But they're a minority.

A really good way to counter this, at least for me, is to keep track of your progress in a notebook or on your phone. It helps a lot to be able to say "oh, I was able to do 2 more reps than average this time" or "Hmm, I've been on this weight for a couple of months, maybe I should go up." Plus it's a good motivator -- it's a lot easier to go the the gym when you can see how much progress you've made.

I think he meant that, if you're not careful, you'll lapse into going through the motions without pushing yourself. In other words, you need to push yourself. And he says being in a group will help you actually push yourself.