What they have is certainly special. Most people who earn $600k want to live a $600k lifestyle. To earn that kind of money, and live so frugally, could be the source of a lot of disagreement in most other relationships-- but money problems in relationships are nothing new.

tl;dr He's a good salesman who was able to pull down $100k/year in his early 20's selling insurance. Good for him for investing and saving most of his income.