When we speak of the dust of the lotus feet of the Spiritual Master, we are speaking of humble approach to serve his instructions. Unless we humbly serve the instructions of the great soul, it is Krishna's arrangement the He never reveals Himself.
By hearing from the Guru, we can understand what is the desire of Lord within our heart.
Worship of The Lotus Feet of The Spiritual Master: There is no work as auspicious as serving the spiritual master. Of all worship, the worship of the Supreme Personality of Godhead is the greatest but the worship of the lotus feet of the spiritual master is even greater than the worship of the Supreme Personality of Godhead. Unless this is firmly realized we cannot understand what saintly association means, we cannot understand what the shelter of a spiritual master means, we cannot understand that we are dependent and he is our maintainer.
A bonafide spiritual master is non-different from Supreme Personality of Godhead. If one cannot say that the spiritual master is the personal manifestation of the Supreme Personality of Godhead, one will not be able to utter the holy name of the Lord.
A guru trains his disciples to become Krishna dasa, a menial servant of Krishna.
Without sadhana, without having unflinching faith in the words of sad-guru, without being disciplined, without being trained one cannot carry out the orders of the Guru.