I love the idea of sharing knowledge and insights, with the goal of teaching what the students need rather than what I know.
I started early on, I must have been 20. So all in all it's been 18 years now.
The pleasure of being able to play music, no matter how simple or basic that might be. Some people need a challenge for their intellect, some never had the chance to learn it as kids and they pick it up at a more advanced age...I think the main reason is still the pleasure of the sound of the piano, which is beautiful and relieving.
The first steps are way easier than most string & brass instruments (no intonation practice needed!). Once the ice is broken though, I think the learning curve is pretty similar all across the board.
It's hard to understand where students want to go with this, sometimes. Whether they'd like or benefit from a certain repertoire, exercise or practice etc. Often times students don't exactly know what they wanna learn (or have a general idea) and figuring out a comprehensive program over a few months might be the hardest part for me.
Basically regular practice! ;-)
The reading part seems tough on most. But it's a very personal matter as well
Change teacher :-)
One should define "proficiency" beforehand. But I guess that 3 years (with regular practice) seem a reasonable time for setting the foundations.
Some are just faster learners and we have to live with it :-) but mostly having a good practice program (I usually make it for my students) can get you a long way