I just wanted to gain a bit of perspective on some ways of answering this (surprisingly tricky) interview question. How would you answer this? What are interviewers actually wanting to hear?
Do these reasons still hold true for those of you still working on your thesis?
As long as you answer in a positive manner, there is no right and wrong answer. Just be honest. For instance, say that you love research, that you love teaching, etc... All they want to see is that you are determined to do the work.
Most tricky questions have direct answers :) Say I want to do PhD because it is required for enhancing skills with detailed knowledge; the word 'specialization' is a close thought to me! And with specialization I am passionately seeking for wisdom.
P.S: (If you cannot convince them....confuse them :D )
Confidence is the key. as long as your confidence is not faltering while you answer before your interviewer, you're alright! so, just say I love the fertile world of my subject. I want to dig deep indeed.that's why PhD.
My reasons for doing a PhD were exactly the raison d'etre for a PhD, namely the chance to do some original work contribution new knowledge to my chosen field.
I would add to that the chance to learn new skills to enhance my future career prospect post-PhD, though that depends upon what you want to do after. Add to that to opportunity to conduct cutting edge science and an enjoyment of continued learning.