Men usually work and carry on their affairs from the ordinary motives of the vital being, need, desire of wealth or success or position or power or fame or the push to activity and the pleasure of manifesting their capacities, and they succeed or fail according to their capability, power of work and the good or bad fortune which is the result of their nature and their Karma.
When one takes up the yoga and wishes to consecrate one’s life to the Divine, these ordinary motives of the vital being have no longer their full and free play; they have to be replaced by another, a mainly psychic and spiritual motive, which will enable the sadhak to work with the same force as before, no longer for himself, but for the Divine. If the ordinary vital motives or vital force can no longer act freely and yet are not replaced by something else, then the push or force put into the work may decline or the power to command success may no longer be there. For the sincere sadhak the difficulty can only be temporary; but he has to see the defect in his consciousness or his attitude and to remove it. Then the Divine Power itself will act through him and use his capacity and vital force for its ends.
Ref: Letters on Yoga Vol. II