"Believing in the mission" requires a suspension of disbelief that can reduce the impact of various factors that would otherwise decrease morale (e.g. lower cash compensation, fewer benefits, unfair working conditions, longer hours, etc).

There's a reason many startups are built on hordes of college kids and it's not that they "have more energy" or "are more willing to think outside the box". They're less experienced and thus easier to manipulate. They're also less likely to have dependents they need to take into consideration, don't understand their limits or trade-offs between short-term performance and long-term endurance (e.g. burnout), and are more likely to be naive about their place in the company and the effect/function of the company. Plus, of course, they're "less risk-averse" which is another way of saying bad at judging the odds of certain outcomes.