Finding a Thesis Topic

Doing a thesis for getting MD/MS/ DNB degree is mandatory.
If you have a good idea of what you want to do your dissertation on, you will get a head start on the thesis, or may decide that you're not interested in pursuing the topic after all (saving yourself a lot of work and grief farther down the road).
Generally speaking, a good thesis topic is interesting to you, to your guide, and to the research community.
To finish quickly, it's usually best to pick a narrow, well defined topic. The downside of this approach is that it may not be as exciting to you or to the research community.
If you're more of a risk-taker, choose a topic that branches out in a new direction. The danger here is that it can be difficult to carefully define the problem, and to evaluate the data you develop.