A lot of people with MA or MSc think they can add a PhD or MPhil very easily-one or two more years of study and that's it. They don't know, however, that a PhD has idiosyncratic characteristics which are entirely different. In other words, a PhD is tough and if you don't have the innner desire to complete it you will soon decide to quit.
It's a marathon and as expressed through this analogy, only crazy people decide to embark on such an endeveaour.