Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 4
The real reason of the difficulty and the constant alternation is the struggle between the veiled true being within and the outer nature, especially the lower vital full of desires and the physical mind full of obscurity and ignorance. This struggle is inevitable in human nature and no sadhak escapes it; everyone has to deal with that obscurity and resistance and its obstinacy and constant 
recurrence; for the lower nature is not only persistent in its repetitions and returns, but even when it is on the point of changing, the general Powers of that plane in universal Nature try to keep up the resistance by bringing back the old movements at each step in order to prevent the progress from being confirmed for good and made final. It is true therefore that a constant sadhana persistent and unceasing is necessary if one wants to go quickly – though even otherwise one will arrive if the soul within has the call, for the soul will persist and after each obscuration or stumble will bring back the light and drive one on on the path till it feels that it is at last secure of a smooth and easy march to the goal.