Hari-bhakti-sudurlabha: devotional service to Krishna is very rare, even more rare than mukti (liberation).
Krishna is attracted to Sri Radha and the gopis because they have bhakti in their hearts. Krishna-akarshani: Bhakti in one’s heart attracts people and even Krishna to him.
Shloka: SB 11.2.39.
Sri Radha’s separation from Krishna. She wants to serve Krishna with Her body and the 5 elements even after Her death.
Prema means meeting and separation.
Lila, when Krishna tried to meet Srimati Radhika by introducing Himself by different names, and She jokingly used these names, thus avoiding the meeting. Finally Krishna pretended to be a cuckoo bird and met Srimati Radhika.
The lila of Krishna as a black swan and Sri Radha as a white swan.