I'd agree that extroverts generally are happier, but I'd argue that a large part of the reason is because the majority of people are extroverts and, therefore, society is built around their personality type. Introverts are inherently at a disadvantage in most cases because they are often treated as if something is wrong with being themselves. The author even suggests that introverts try to gain happiness by aping the behavior of those who already have it, essentially saying that if you're not an extrovert, then your only option is to turn yourself into one.