I don't agree with the do what you love thing always. For some people I've met, they have to. Some of my friends that were artistic had to play music, some started a band and some majored in music education. I don't think they could have survived if they didn't play music.
For the rest of us, I think you have to follow your interests and talents. Most engineers I know liked math and logic related courses better than say English or history. So they followed their interests and talents to engineering. If someone isn't gifted in that relm, or they don't have a genuine interest, then they're destined to fail.
Frankly, I made a huge mistake becoming a pilot after college. I don't regret it, but I had a feeling at the time that I really didn't want to do this for a living. I had watched my dad work 3 days a week and earn $300,000 for my entire childhood, I also had dad issues and desperately wanted to please him (though it took therapy to figure that out). 5 years later I quit. I find that if you don't like what you're doing you probably won't be happy, or very good at your job.