First off, if you aren't reading SquatRx, you should. What Boris posts is always eloquent, effective, and interesting.
"Try to Never Send a Loser Off Your Training Site"
Boris quotes Lt. Col Dave Grossman w. Loren W. Christensen's book On Combat about teaching success vs. inculcating a failure response, with a side order of avoiding demonstrating the superiority of the teacher instead of teaching.
This dovetails nicely with how I train, both myself and my clients.
My goal as a trainer is to send you home with a "training effect" - by which I mean, enough stress that you will adapt positively from the workout. A workout that is enough to make you better after you rest.
Also, if possible and appropriate, I'd like to send you home some kind of improvement within the workout - a personal record, a new skill, a harder variation. If possible - if it's more appropriate to give you a lower workload, save PRs and new exercises for next time, or otherwise dial it back, I'll do so. The goal is results, not showing how hard I can push you.
It's easy to find failure - weights you can't lift, exercises you can't do, workouts you can't complete. But it's hard to find just enough success to make you better. There is a line between "not enough" and "too much" and my goal as a trainer is to find it and have you train there, and adjust it each and every time to stay on the path to your goals.
I don't care too much about sending you home tired, worn out, or wrecked. This is not to say that this won't happen. You may be briefly crushed by the Thomas Finisher or your arms or legs might be rubbery from 100-rep sets. Some workouts will take a lot out of you. But not all of them, and it's a side effect not a deliberate, workout after workout effect. What matters is what you get out of it in the long run, not how hard it feels like you worked this workout. Like Boris says - failure breeds failure, and success breeds success. Work hard, but more often than not you want to take success home with you from the gym rather than failure.