Startups are real companies. They are not the comfortable lazy jobs you would have on a big tech company campus or even a post-IPO tech company. They are still trying to "change the world" or else they figured out it's not going to work. The founders typically leave within 5 years of starting them, either because they succeed and get bought out or because the investors stopped financing their continued existence for a particular reason. Maybe the idea was great at the time but the market changed. Maybe they were wrong about the idea from day 1. They are often 1 mediocre executive decision from going out of business.

Big companies are full of people who aspire only for a comfortable existence in a relatively easy job offering a career path or a lucrative compensation package that will get them the nice 2 story home in the suburbs and pay for the kids private school this year. They work a standard 40-50 hours per week unless they're on the management track and then they may leave the baby at home with the nanny until they make Vice President of something very ordinary and mediocre.

In a startup, depending on what it is, you could be working for or alongside the next Steve Jobs and not know it yet. You could also be working for and alongside total bozos. Figuring that out is key to knowing whether its worth your time. Steve was kind of an a-hole according to some, so it might not be apparent until you talk about serious things having to do with the vision for the future. You will probably have 5 to 10 jobs that change frequently and will work way more than a standard workweek.

It's like saying "I'm going to join the military and can either become a supply clerk or join an elite unit whose name is not publicly acknowledged". Most people should probably stick to supply but the others will never be fulfilled there.