"More than 25 percent of graduate students quit after being advanced to candidacy and before submitting a thesis proposal. They simply can’t find an interesting topic that passes all tests of feasibility, originality, and contribution, and which matches their resources, aspirations, and abilities. Many students searching for thesis topics become discouraged and begin boomeranging from one idea to another. Professors naturally question these students’ stability, reliability, or ability. Advisors may conclude that these students can’t stay focused long enough to complete a proposal and see it through. This negative view may cause advisors to withdraw support, consciously or unconsciously. Without a viable topic or a supportive advisor, students give up and drop out.
To help students avoid the pitfalls of interminably tangling with an ever-more-elusive topic, many professors advise students to simply join an ongoing research effort in which topics are generated by the advisor. Indeed, this is the common research model in the hard sciences. There are, however, important reasons why this may be poor advice. " -- http://www.asgs.org/