I am of the belief that most people are too young to choose their career paths by the age of 18. In spite of this we tell our young to start thinking about their careers from they time they are 14. Adults convince themselves that they leave their kids free to choose absolutely anything, yet they will persistently throw in their two cents worth along the way. As a teenager you may mention that you are intrigued by bio-medical science, for example. Your dad replies, trying to be diplomatic: "Hmm. There is not much scope for that, there aren't many bio-medical companies around and they don't pay so well...My friend is an investment banker and gets to go overseas four times a year. He lives like a king." So substitute bio-medical with investment banking.
Grownups unwittingly coerce their young into choosing what they think is the right choice. In the end you are left with a list comprising of law, medicine, accounting, banking and mechanical engineering. Everything else has been duly ruled out for you by the wise elders. But you still have a list of the above choices, so they pat themselves on the back believing they are leaving you to choose "whatever you want." Whether their kids are remotely interested in banking or medicine is not of importance. In turn kids listen to their parents because as a kid you believe that the elders know what they are talking about. I mean, what do you as a teenager that's never worked a day at a real job know?
So at 18 you choose your tertiary path as advised by your wise uncles and parents. Not once did any of them ask you where your interests lie, but you trust in their wisdom. Anyway, at that age you think it's all about the money so you go with it. You slug it out for three, four, five years, get your degree and join the working world. Then it hits you like a train: you can't do something you aren't interested in for eight hours a day, even though it earns you a salary. The people that advised you on your career aren't there to shoulder the stress, frustration and boredom you experience in a typical working environment. Finally it catches up that you didn't consider the most important criterion in choosing anything: You.
"Fine," you say, "ultimately I did choose this path, but I can still change direction and start doing what I actually enjoy." Suddenly out of nowhere the grownups intervene. They want to know what your plans are exactly and how you know whether it will work out. "What will you do to earn an income? Is it guaranteed?" You say you aren't fully sure of the details as yet, but you know that you have to leave your boring desk job because it's eating your soul. They then retort with the world famous "that's life." Finally they bring in the fact that you studied for four years and if you change the way you earn your livelihood, all that studying you did at university will amount to nothing. So they are effectively telling you to keep at something you hate for another 40 years because you spent four years studying to do it.
This is where you need to be headstrong and ignore everybody's advice. They are not there to help when you are taking orders from a creep of a manager or dealing with eight concurrent deadlines. Therefore their input counts for nothing. If you want to change direction, even if it's a 180 degree turn, do it.