Sraddha increases only by association. Srila Bhaktivode Thakura gives the analogy that sraddha, when appearing in the heart of the living entity, is like the birth of a new-born baby girl. When the baby girl takes her birth, her parents are always careful to protect her from severe heat and cold, hunger and thirst, and dangerous creatures. Similarly, sraddha must be protected from the influence of karma and jnana by the guardians of devotional service. If not properly protected that sraddha, instead of growing into more advanced level of faith, can turn into anarthas. Therefore sadhu-sanga is essential for protection of sraddha. And along with protection, the new-born baby girl requires nourishment from her mother. Similarly, sraddha requires tnourishment from the hari-kirtan of those who are also protecting their sraddha from bad influences.
Collected Letters Written to Friends and Disciples Vol 4, Pg 44, 2nd paragraph