Why did you work on your PhD?
- Is it because you had a passion?
- You had curiosity you had to explore?
- Did you want to work with your supervisor and together steered things to the work that became your PhD?
- Or did you apply for a project that seemed interesting and you thought would suit you?
Any of these situations are fine. There’s no magic “best reason” for doing a PhD or selecting a topic. “Why did you do it?” is a good starter question. Whatever your reason is it’s really a lead-in to a topic you’ll definitely need to talk about in the viva:
Why was your research worth doing?