Hakim al Umma Moulana Ashraf Ali Thanwi (Allah have mercy on him) said,
It is absolutely vital for one to constantly remain vigilant and cautious regarding his nafs. The nafs is such that if it finds the opportunity and the means to fulfil its desires, it will never rest until it gets what it wants.
In fact, even those who have progressed sufficiently in the path of self reformation, it is nevertheless, dangerous for them as well to become complacent and relax their guard regarding the nafs. However, the difference with them is that due to their knowledge and experience in traversing the path of self reformation, it is not as difficult for them to bring their nafs under control at the time of danger.
The condition of our nafs is like that of a stubborn horse. When it is let loose, it neither looks forward nor backward (as it is out of control and runs in any direction without thinking). In this state, the damage that the nafs can cause is so serious that we cannot truly comprehend it.
(Malfoozaat Hakeemul Ummat 8/50)