Its rather insufferable when youve to hang around with a Negative Nelly who always assumes the worst and has zero optimism but gather up your spirits because the Fourth Doctor doesnt take no for an answer. Romana was a really difficult person. Choosing to take a bath at impossible times, stealing someones body and taking it as her own, killing the joy out of everything... well, you get the idea. The Doctor just couldnt stand the constant drill of her lack of enthusiasm when she would instead believe in a huge nuclear war than a huge breakfast party going on in the street because her optimism, believe it or not, opted out. When youve got someone whod always assume the best with every situation instead of waiting to see how it goes, you now have an ingenious insult in hand, with your irrationality bordering on insane.