I am just speaking from what I have experienced with my clients as a personal trainer. I have clients that I see on a regular basis 2-5 times out the week for months straight and then I won't hear from them for another 6 months or so. And yes, what we have worked on so long and hard for has returned. Listen people...the more you keep gaining and losing weight, the harder it becomes each time to lose weight. You want to stay consistent with your program and don't be afraid to ask for advice on changing things up.
There are many reasons why people yo-yo exercising:
1. Work
2. Bored
4. Lazy
5. No results
6. Family obligations/Relationships
7. Holidays
8. Travel etc
I mean the list can go on and on
Basically to sum up what to do about yo-yo exercising is to plan a realistic schedule whether its 1 day a week or 3 times a week and stay consistent with your routine. If you get thrown a curve ball in life, try your best to get back on track and continue. You can also hire someone to help you with a more aggressive program if you need the accountability. Stay motivated and consistent!!!!