I went into college really interested in chemical engineering. I graduated with an engineering degree in computer science. I thought I really loved writing software. After four years, my skills in writing software have increased a zillion fold and the excitement has faded.
I don't think everyone is lucky enough to pick a great job as their first 'real' job. The moden approach is to move through numerous careers.
I'm working now on building the cajones to switch careers. It's a big risk and scary as hell to leave the security blanket of my current job.