Example 1: Say you love animals - and you decide you want to become a vet. That takes a LOT OF TIME, MONEY AND COMMITMENT!!! Then, you finally finish school, 250,000 dollars in debt, and realize you cannot bring yourself to give an animal a shot or perform even minor surgery.
Example 2: You love languages. You decide you want to be a foreign language assistant. That sounds great - like it might involve travel, fascinating meetings, hell you might even be able to interpret cool-ass, confidential things. Then you realize you basically just answer phones and write e-mails all day, and the most common question you get is: "WHY AREN'T MY GOODS HERE YET!!!???!!!"
Example 3: You want to become a doctor because you want to help people and you happen to love biology, chemistry, physics, and math. You are a genius when it comes to theory, getting all As in your classes. Looks like Pre-Med is a shoe-in for you; only to realize you cannot stand the sight of blood, urine, pus and feces. Hmm, that might be a problem.
Example 4: You want to earn a hell of a lot of money in a short amount of time; you are a genius in figures; you are intuitive; you can sense market trends before they become true, so you become a stock market trader. Only to find out that you have 80-100 hour weeks and do not earn nearly as much as you thought for this cutthroat, STRESSFUL AS HELL job.
Etc., etc., etc. Of course, these examples are extreme and highly exaggerated to prove a point. But you get what I mean, right?
Excellent advice, Cindy.