lots of truth on here,the bottom line is don't change,you think you feel bad now,how you you going to feel when you don't give your all to your students
Everything you do will be remembered. I train at a great judo club but had to give up Judo because of injuries. The instructors are great and I ended up enrolling my kids there. I am always recommending them to my kids friends etc and have sent them a few new members.
The transient nature of bjj is just the way life is now. People rarely live in the same place for long, or keep the same jobs or hobbies for long. 50 years ago people would have the same jobs, hobbies, friends etc for life. Now people are constantly moving, changing jobs, so their availability for hobbies constantly changes. My wife and I have felt the same way about putting in the effort to make friends in our neighbourhood only to see most of the people move away after a few years. No one stays in the same place, job, hobby for long anymore. Kinda sad but that is life now.
Not an instructor but a long time student. The social/emotional attachment is similar. There is a reason that it can get cliquey in BJJ gyms. I always try to be nice and say hi to new people, make them feel welcome - but I try not to get too attached as chances are they won't last.
Treat it as a martial art and not a Business. Teach the art and don't concentrate on the individual student. 
Honestly you need to figure out what the majority of students coming through your door are looking for. Obviously its BJJ but lets be real. Jiu Jitsu is a tough sport. Majority of people that work 9-6 are not going to last long if the training is rigorous. When someone comes in you should have a small QUICK questionairre and pinpoint what it is they are looking to achieve. If the majority says weight loss you add more drills to the class, self defense add a bjj self defense class etc. Gotta give people what they want. At our academy we have implemented a new schedule and its working well for us monday, wed, friday is a regular class (warm up, technique, drill, Spar) Tuesday and Thursday is Drilling only (warm up, drill chains of techniques)Open mat after for those who wish to stay and roll but You would be surprised how many people show up for the drilling class and how well their technique has gotten