When a person begins to do something with Śraddhā, with conviction, why does this very often dissipate?
Response from TKV Desikachar:
Because Śraddhā influences the mind from within. But the mind has also to continually interact with the external. In the course of these interactions, the mind will often come in contact with situations that evoke other responses that are contrary to the original conviction. When the Śraddhā, conviction, is not strong enough, the mind will follow the contrary response. Over time the original conviction may even be forgotten.
A person may want to keep his voice in good shape and for this reason decide to stop having ice-cream. He may even faithfully follow this for a while. Then somebody brings home a carton of a new ice-cream recently introduced, of a quality never eaten in India before. His interest is aroused and he finds many reasons why the ice-cream can be had: ‘the friend has gone through so much difficulty to get it for rne’, ‘it is a special occasion and the whole atmosphere of the occasion will be spoilt if I refuse’, ‘I will just sample it once to know what its like’ and so on. The voice is forgotten, and the person eats the ice-cream thereby strengthening the contrary response and weakening the Śraddhā.
– TKV Desikachar on Śraddhā in the Yoga Sūtra