My first job in aviation was flight instructing. Reflecting back on it now you come to some insights that weren't apparent at that time. Overall I think a job is a job after all, when it becomes routine and too much of the same thing happens it has the potential to become boring. I was fortunate to do a little bit of everything, PPL, IR, ME + ground instructing, administrative etc. The variety kept me going for 3,5 years. At the end I realized my own limitation, and as I was getting tired of it (which easily happened after 4 flights of steep turns in 35 deg heat) I decided to move on. The students also deserved better. They needed a mentor, someone to nurse them through a demanding challenge, rather than someone who came to work, did the job, took the money and got the hell out. At the end, I did not feel I was that person.
Don't get me wrong though, I totally enjoyed instructing for several reasons; To share knowledge and experience, To grow as a person myself and develop my own abilities, Not to mention the times you took a C172 or BE76 to Santa Monica, Las Vegas or Catalina Island for dinner and come back. You could have so much fun with the students while also conducting serious flight training.
Let's face it… the vast majority that starts in aviation aim to one day fly a big shiny jet. Not necessarily everyone, but most... I have a friend that spent €70 000 - 80 000 on training, working as an FI and says he's happy and have no motivation to get into the airlines he says. But for most FI is a transition job, a way to gain experience, it doesn't pay enough to be a career choice either.
Today I wouldn't mind coming back doing instruction, if not in a C172 I know the airline does train simulator instructors to do initial type ratings. As someone said before, you never really know how to fly untill you have taught someone else how to fly.