What do you answer 'Why did you quit your previous jobs' in a job interview?

The best answer that you can give is -

I quit my previous job because I could not find ways to improve my self further and expand my knowledge in the field that I am working in.

I applied to this job because I found that it had the potential of improving myself. Added to this I can enrich your company with my experiences.

Another option: I was no longer comfortable with the decisions being made by the management and the direction in which the company was moving.

It is normal nowadays to expect that an employee will work for many different companies over the course of a working lifetime. But it is never advisable (though it may sometimes be necessary) to leave a job without having another job - preferably a better one - to go to. Extended periods of unemployment create a bad impression in a CV and do not give prospective employers confidence. It is always easier to find a job if you are already in steady employment (even if it's flipping burgers) than if you're unemployed - for one thing there is no break in your income stream.

If you're serious about a career you should always be looking for ways you can learn more, broaden your experience, develop personally and express more of your potential. Even if you're in your dream job now you don't know what lies ahead so be prepared.