The Doctor was given his nickname, I believe, not so much because of his medical degree, but rather because of the way in which he used his diagnostic skill to get to the root of the human predicament. This quote is a good example of that:

To fail to control ourselves means a loss of energy. These things can actually be measured. When I say ‘self-control’ I include controlling one’s temper, controlling one’s spirit. Have you ever seen a man trembling in a rage? What energy that man is wasting! He is emitting energy at a tremendous rate because he cannot control his temper, and his own spirit. And, of course, he is but as putty in the hands of the devil. When a man cannot control himself how can he possibly deal with the enemy? Discipline is an absolute essential in an army; it is one of the most important things of all. If an army is not disciplined it is already defeated, it becomes a rabble.

The Bible has much of this kind of teaching. It is a major theme in the Book of Proverbs. The wise man in dealing with this matter says: ‘He that is slow to anger is better than the mighty’ (Proverbs 16:32). What he is saying is that a man who is slow to anger, a man who can control his temper, is a much stronger man in the end than a mighty man who loses his temper. The second man is much mightier by nature, but if he dissipates and wastes his energy by failing to control his own temper he will lose the battle. This first man has nothing like the vital force and capacity, nor the strength of the second, but he controls himself; and a man who can control himself will often beat a man who is very much better at the task, and who has much greater strength, simply because he is reliable and steady.

David Martyn Lloyd-Jones, The Christian Soldier: An Exposition of Ephesians 6:10 to 20 (Edinburgh; Carlisle, PA: Banner of Truth Trust, 1977), 101. Also available electronically from Logos Bible Software.

NOTE
This photo of "the Doctor" is quite rare, according to Philip Eveson, principal of the London Theological Seminary, where this portrait hangs inside the Lloyd-Jones library. Although pastor of the Westminster Chapel in London for many years, the Doctor was originally born and raised in Wales, and he also pastored his first church in South Wales.

For more information on Dr. Martyn Lloyd-Jones, see this summary post or the MLJ Recording Trust.