I would also add that stock options are not stock grants. You are given the option to purchase those shares. Meaning you have to pay the company for them at strike price within 90 days of leaving and you have to pay tax on them.

Many young startup employees I talk to are under the impression that they own .x% of the company because their options have vested. That is not true.

After exercising your options the company could shut down or it can be acquired with a valuation less than what your options and strike price were based on. In both of these cases you would be losing money.