There are probably as many views about choosing a topic as there are research supervisors. What follows has been "road tested" at a number of different universities.
The topic you opt to work on depends a lot on the type of research degree you are planning to undertake. At the outset most students, for most research degrees, imagine working on a topic that generally is the size of 5 or 6 PhDs! Don't worry. Solving the educational problems of the world is a good aim. But, as your supervisor will remind you, you have to do it in a limited timeframe.
There is also a kind of romance about working on a problem that you have been thinking about for a long time.