Why do a PhD? For the money? This is one reason, for some it is a very good reason. But for most this cannot be the main reason. Why not? Well, a PhD means 3 to 5 years of sweat, tears and absolute commitment. 3 to 5 years in which you will feel the pressure of PhD every time you take a bath, drink a coffee with a friend, or watch TV; with other words: every time you do something other than PhD work. For most, the increase in income following a PhD is not enough to make up for the loss of 3 to 5 years of your life.
So, what else might motivate you to do a PhD? A thirst for knowledge, a desire to ask the next question, a joy in the multitude of perspectives to look at what you find. But even that is not enough. That enthusiasm will only get you two months in before the newness wears off and the boredom of the research routine, the need to concentrate on the minute details of your research and the sheer frustration when you, again and again, realise that whatever you wanted to do will not work.
Therefore, it is not money and it isn’t the thirst for knowledge, that gets you through your PhD – it is your capacity for sheer, unbridled stubbornness. The obstinate refusal to give in. That part of your mind which is simply unable to accept, or even contemplate, that you might fail at it. Without any of the other things, you would not even start a PhD – without the grit to work against all obstacles, even yourself, you won’t make it through it.