Someone recently asked me how I became a technical writer. She was most surprised to hear it was via "cheese-maker", "secretary", "admissions officer" - and a whole pile of blogs, articles and other writing stuff thrown in.
Gone are the days of going to uni after school to study one thing, starting a career in that area and doing it for the rest of your life. At the most you'll go to uni, study one thing, and then move within related fields for the rest of your life as the world changes. Computing recently mentioned in passing that over a three-year period most new recruits to an IT company will come and go three times. Technology is on the move, life is on the move, and it's no longer a requirement to retire from the same job you started at when you were a pipsqueak.
So changing careers and fields of interest completely a couple of times isn't really out of the ordinary. Yes, it may mean you're never an expert in one thing, but it also means you don't stagnate or lose interest in the job that takes up the majority of your daily hours.
I guess it's called going with the flow.
The flipside is that some never find direction. They never commit to one thing fully - knowing they'll be moving on in a year, a few months, as soon as they lose interest in what they're doing.
But on the other hand it makes for constant growth if you approach it in the right way. Each new job has the potential to expand your horizons, stretch your mind and make you better at what you do. You have the chance to throw yourself fully into a job until you've exhausted all its facets - and then move on. Each career shift has equal potential to make you a better person, a more useful member of society, or to find you that niche where your passions can take flight (profitably, we'd hope).
I'm all for going with the flow, using the opportunities and open doors life throws your way as they arise, adjusting plans and goals as needed to seize chances as they pop up. Change can be difficult, unsettling and unsure at times, but I've learned that it's well worth it in the end if you choose to embrace and not fight it.