Some people thrive on a vegan diet and some more on a paleo diet. why is that?

Need more info: Thriving is related to many factors. All things being equal, some people do better on one vs the other. How long have the individuals been on either diet. How much do either exercise. What is weight, height, bmr, other health issues. There is more long term research into the benefits of vegan vs paleo. You need to be strict on either regimen to see what is better for you, not short term.

Sugar: The reason you lose weight on either of these is that you aren't eating as much sugar generally. Without sugar you don't spike insulin as high and when you don't spike insulin (the master fat storage hormone) you lose weight.