The Master reacting to Dr Who's sarcastic quip
It all dates back to The Master's attempt to get a job at a prestigious college in the USA.
Despite a dazzling curriculum vitae of graduating with first class honours, and vast numbers of publications (hand written papayri, parchments, printed books and electronic micro-podules), and teaching experience stretching across several millenia; his application was rejected without being sent to referees on the basis that The Master did not have a PhD.
The Master had, in fact, dropped out of the University of Gallifrey after completing his MA; and gone off on a Galactic Grand Tour while laughing at his old friend Who who had to grind away at mundane research for a further three years towards his doctorate.
Instead of showing sympathy at this humiliating injustice of rejection on an academic technicality, The Doctor (as he now styled himself) snidely remarked that this was exactly why Who had finished his studies instead of chasing-off after stars, seas and sensuality, leaving the job half-done.
The Master was, naturally, offended and angered by Who's arrogance; and spent the rest of his life chasing The Doctor around space and time, hoping to torture him to death as pay-back for that wounding remark.