1. You fell into your current career. Your current career developed through a number of (perhaps random) twists and turns and wasn’t necessarily something you applied conscious decision-making too.  You now feel you want to do something more suited to you.
2. You want make better use of your skills.  Your current career doesn’t make the most of what you can offer.  You know you can do better and go further in a different field.
3. You need new challenges at work.  Perhaps you’ve reached a dead-end.  Perhaps you need a steeper learning curve in order to feel fulfilled.
4. Your old career doesn’t suit your changing circumstances.  You’ve had a baby.   You’ve moved to a different part of the country and, thus, a new local labour market.  You’ve won the lottery and now want to do something for the love rather than the money.  There could be 101 different reasons….
5. You don’t suit the changing circumstances of your career. The future of your industry looks bleak.   Your company has restructured and it doesn’t suit you.  Personnel changes have affected how you feel about your job etc…
6. You need a bigger earning potential. Enough said.
7. You’ve always ‘had a dream’ and now’s the time to live it.  You’ve spent the last 25 years harbouring a dream to be a wildlife photographer/inventor/ astronaut.  It’s now or never.

So ask yourself that all important question: “what do I really want to be doing in 5 year’s time?”  And be realistic about what you may need to do in order to get there.