The reason why it is important to be proactive is to stay employed and to not end up being treated like a child. If you are not proactive, it would be a rare company that considered you for a promotion as well. People who are not proactive rarely get the best assignments either, they get the work no one else wants to do.
Managers don't want to waste their time treating someone like a recalcitrant 5-year old. Far better to let that person go and hire someone who is not going to make your life as a manager more difficult.
It is always part of your job to make your manager look good. When you are reactive instead of proactive, you make him or her look bad or you consume more of his or her time than necessary and thus do not allow him the time to do more important tasks. This tends to make managers cranky and, well, why keep someone who is causing you to have to do more work?
You can get away with this when you are trainee or if you work a minimum wage jobs where you aren't expected to think, but if you have more than 2 years experience in a professional field then you need to start being proactive or you will be stuck in an endless series of bad jobs. The interesting and challenging professional jobs require a person to be proactive. The boring ones where you work for a micro-manager do not. Perhaps you don't want to do anything at work other than obey a manager. If so, you are likely in the wrong profession.
I had a subordinate who used to drive everyone crazy because she would follow instructions to the letter. That meant if you didn't actually say to her to let someone know when she finished a task, she would not. Everyone in the organization hated working with her. She hated working for me because I wouldn't let her get away with that behavior and I specified to the tiniest detail what I needed. I got four times as much work out of her as anybody else, but we both hated every minute and I was happy to pass her along to someone else as soon as humanly possible. No decent manager I have ever worked for or with wanted an employee who is passive. So when they find out that you are they take steps to move the problem to another manager or company.
It was a shame because she was a bright capable person capable of doing so much more (the work she actually did was excellent in quality) and she did herself in with her attitude. She was never considered for any good assignments, she was never promoted, she was frequently called into the big boss's office for performance discussions. She managed to stay exactly on the line of the performance that would not allow us to fire her (there are more rules for this in most government agencies) but she would have been fired from most corporate jobs. This is basically the path you are on.
Your job is much more than technical. Technical skill accounts for only around 30-40% of your success at work. All jobs require some level of interaction with your boss and coworkers and most professional jobs require some level of personal initiative.
If you are quiet and reserved, too bad so sad, but that is irrelevant. You still need to gain these skills to be successful at work. Every quiet and reserved person I know has managed to get those skills and you can too if you try. If you don't want to try, then you will have a very unfulfilling career and probably get let go multiple times until it becomes harder and harder to find a new job.