You should consider your interest, experience, expertise, and resource available. One way to pick a topic that will remain exciting beyond completion is to pick one where your writing will have a social impact (as well as personal interest). But you should do some preliminary research to see whether you can find sources you can cite to make your thesis acceptable, whether the topic is too broad or too narrow, and so on; in that process, you will also be able to develop your intellectual position for pursuing the research and framing your writing. Hope that helps.