Why do a PhD?

Inevitably, the question always arises from family and friends, "Why are you doing a PhD again?" Well, now you simply have to choose a category that you fit in and tow the party line says an article from The Guardian. Are you doing your PhD because: a) You are genuinely interested in your chosen field, b) You want a higher salary and or/job advancement, or c) Prestige. Categories make things so easy.