One is that if you work hard at school, get the grades, go to a good university, get a really good job, your friends and family will be impressed, and you will live happily ever after.
The higher up the ladder, the happier you will be--another false assumption. If you fall off the ladder, you will be miserable. Yet another.
Parents want this because they want you to be safe. The companies create great work environments to get the best people or what they consider the best people.
After people have been in the working world awhile, they have moved past some of these assumptions--have seen the exceptions everyplace.
Some of these exceptions can be found at "big" companies. So they leave.
--They don't love the politics of a big company.
--They have an idea for their own company.
--They want to pioneer a new technology.
--Family reasons--a spouse's job, a sick parent.
--They want to break out and travel while they are young.
--Or maybe they get a taste of technology and don't love it and want to become something else.
But if you bought into the false assumptions--leaving a big company can throw you into an identity crisis. It can cause problems with family members--you want to become a WHAT? They may also question their decision-making--this company was supposed to be perfect.
Or maybe because of the false assumptions, they don't leave. They become mired in inertia, doubt, desperation.
There is no "best." Only "best for you."