Does being strict work?

I have been teaching piano for a few years now. I have always been of the idea that learning can only happen if the student is genuinely interested and wants to learn. Even the adults that I have, some of them want to learn a specific type of music rather than what I initially propose them (and of course we do end up working on the music they like). To me the learning process is always centered around the student.

In general I'm not much a believer in the concept of "discipline", and I believe that people do the things that they want to do.

If a kid shows up a few times to lesson without the books, I think "They don't care about piano lessons. Can I do something to motivate them and make them interested in learning?".

The same reasoning applies when they often miss lessons, or they haven't learned primer pieces after months, or if after a couple years of lessons they still need to count the notes from C in order to find F. This tells me that they're not interested, and scolding them might make them quit.

However I am wondering if I am being too lenient with my students and if being stricter would produce better results. What's your take on this?