I think we all need that kick of motivation to hold us on track. For my mom, she was having trouble with her knee and was facing a possible replacement. Her sister had died after a long series of complications that started with her knee replacement, so mom was not interested. Now that she has lost her weight her knee is only an occasional minor nuisance, no longer a constant pain.
For me, I want to be an active grandmother (hopefully I have more than a few years before that is actually the case).
I think it has to be about more than the number on the scale or I would have quit a long time ago (which given my yo-yoing might have been better for me

)