How do you motivate teenagers? When do you just "allow" them to fail?

I teach high school. My philosophy thus far is that I cannot make you work…if you choose not to do the assignment then you will reap the consequences. I do this because I want to prep them for life, where no one is there to “make” you do things… and I don’t have time to hover over them anyway.
What is your opinion on this? For those who don’t EVER do their work, what is a motivation? (Slapping isn’t allowed, I checked)