I would say financial reasons in 90% of cases if it is to leave Vipers altogether (moving to a newer model wouldn't count to me, although many would argue newer is not necessarily better in many respects, some love Gen I, some love Gen II ie, me, and some love SRT-GenIII).
Other than those, the odd health or "family growing" concerns,this is a pretty passionate group here, at least those that are reading and responding to your thread. I have some local Viper owners (mostly mothballed on jackstands) that do not have a clue what the Viperclub is nor this website. To those it may just be a car that is casually flipped to something different when boredom sets in. Personally with 500 horse boredom has yet to been a cause for selling, although a new hobby of making it faster does tend to sprout up quickly, you'll want to budget for that!
I just met up with the Florida VCA crew last week, didn't bring my Viper, met at least 20 people that I knew from this site only. Talked cars for a while, drove around in them, went for wings and off to a ********** and some bars. Not sure that would happen if I bought a Lamborghini (which will happen), the love of this car is a little bizarre at times, and I'm in the middle of it.
Go get one,