if you accept a large portion of your compensation in the form of stock, then you are pretty much a de facto believer.
Selling my “equity” in a private company once it vested wouldn’t have been an option.
Note for those who don’t know: Amazon has a 5/15/40/40 vesting schedule with their first four year offer. The first two years you get a large pro rated cash signing bonus to make up for the back heavy vesting.