Your motivations should evolve over time. You may want to lose weight to enact revenge against someone, but will that enable you to maintain for 20+ years? Will you even remember that person's name after all that time?

Anyways, everyone tends to have a boiling point where they want to change the direction of their life. Someone crossing you is as good a point as any.

I don't think there are good and bad motivations. Each of us is different and what works for me may not work for others. My health is pretty good, so I don't have any particular health concerns for losing weight. I just want to lose weight to look at myself in the mirror and like what I see, to feel that what I see reflects the beautiful person I'm inside. I know it's silly and kind of shallow, but that's my motivation at this point. To love myself and gain the self confidence I never had but I know I can reach through weight loss, because I was getting there last year when I lost 40 lbs. So... there. This might not be a good motivation for someone else, but it's the world to me!

To some extent I also believe there are no right or wrong motivations, but I think if your motivations are suspect (like in the OP's case of doing it for spite) then you may be in more danger of regaining.

I think as long as the weight loss motivation takes on a new life of its own even after the original motivation then the original doesn't really matter. This is sort of what happened for me.