Out of many millions of wandering living entities, one who is very fortunate gets an opportunity to associate with a bonafide spiritual master by the grace of Krishna. By the mercy of both Krishna and the spiritual masters, such a person receives the seed of the creeper of devotional service.
The Lord is always worthy to be the master of the surrendered soul, for He is the ocean of mercy.
Srimad Bhagavatam 4.8.46,
A devotee is always begging the Lord for His mercy.
Krishna does not agree to award devotional service to merely anyone. Krishna can easily offer a person material happiness or even liberation, but He does not agree very easily to award a person engagement in his devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee. — The Nectar of Devotion, Page 14.
When we face difficulties and still remain faithful to Krishna and the spiritual master, we access the infinite mercy of their hearts.
To travel in another country, you need visa permission, similarly to go back to Godhead you need permission & mercy from his pure devotee.