In the preliminary phase of spiritual life there are different kinds of austerities, penances and similar processes for attaining self-realization. However, even if an executor of these processes is without any material desire, he still cannot achieve devotional service. And aspiring by oneself alone to achieve devotional service is also not very hopeful because Kṛṣṇa does not agree to award devotional service to merely anyone. Kṛṣṇa 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. In the Caitanya-caritamrta it is said: “By the mercy of the spiritual master who is a pure devotee and by the mercy of Kṛṣṇa one can achieve the platform of devotional service. There is no other way. ”
Characteristics of devotional service are described by Rupa Gosvami with evidences from different scriptures. He states that there are six characteristics of pure devotional service, which are as follows:
1) Pure devotional service brings immediate relief from all kinds of material distress.
2) Pure devotional service is the beginning of all auspiciousness.
3) Pure devotional service automatically puts one in transcendental pleasure.
4) Pure devotional service is rarely achieved.
5) Those in pure devotional service deride even the conception of liberation.
6) Pure devotional service is the only means to attract Kṛṣṇa.
Kṛṣṇa is all-attractive, but pure devotional service attracts even Him. This means that pure devotional service is even transcendentally stronger than Kṛṣṇa Himself, because it is Kṛṣṇa’s internal potency.
Source: A.C. Bhaktivedanta Swami Prabhupada (2011 edition), “The Nector of Devotion”, Page 3 & 14