If you're instructing just to build time and move on you don't need to enjoy it; you just need to tolerate it.
That said, regardless of whether or not you enjoy it you owe it to the students paying you to give it your best.
Plus that person you trained a few years ago, might be the Captain on the regional jet you had to fly to go to Podunk Nowhere for a family funeral. Hahaha.
Kinda kidding, kinda not.
Don't half-ass teaching. It might get you or someone else killed. Or a whole lot of someone else's.
Plus, let's get more real here... the biz is really small. If you half ass someone's instruction, eventually they'll run into someone who'll have to fix it and they'll ask who the instructor was who taught them that crud.
Word gets around about time building instructors who are just phoning it in. Really. It does.
It's bad enough to be a weekend warrior and know you're being scrutinized. Crusty old chief pilots at clubs love to make fun of the CFII who hasn't taught a Primary student in years and has a constant stream of Flight Reviews that never seem to take longer than the prescribed exact 2.0 with 1.0 on the ground.
Whether deserved or not, people talk. Just sayin'. The best anyone can say of anyone in the training side of things is that they know someone and they're "tough but fair".
I think to be "tough but fair" you have to out in some effort. Finding tough stuff that just isn't loading someone up with workload for no good training reason, while still making it a fun experience, for experienced aviators, is work. Real work.
Unfortunately from what I've heard, too many pilots show up unprepared enough that finding their "tough" is easy.
And of course, I'm a total noob so I'll be the guy under the microscope for a while by the instructors who know how to do it well. I don't mind. Would rather that than think the job was easy.