The addressee of the poem is God. The somewhat timid translation obscures this, but it's pretty clear in the original Bengali.
As Kumud Biswas's note to her translation says, the original of "Cruel Kindness" is poem 2 in the Bengali collection গীতাঞ্জলি / giitañjali (1910). This poem was not included in the English collection also called Gitanjali (1912). The poem begins:
আমি বহু বাসনায় প্রাণপণে চাই,
বঞ্চিত করে বাঁচালে মোরে।
এ কৃপা কঠোর সঞ্চিত মোর
জীবন ভ'রে।
aami bahu vaasanaay praaNapane chaaii
bañchita kare baa.Nchaale more
e kR^ipa kaThor sañchita mor
jiivan bhare.
A literal translation would be something like:
I desire many things lustfully and ardently. By depriving me of them, you save/preserve/rescue me. This cruel mercy fills my entire life.
The poem continues through a series of paradoxes that elaborate on the "cruel mercy". God has given him many gifts without his asking: sky, light, body, soul, and life. These keep him from wanting too much. When God has (apparently) turned away from him, it's in order to draw him closer, because the suffering the poet undergoes in such moments perfects him for ultimate union with God.
It's worth noting that, unlike what one would expect in a Christian context, lust is not considered something to be overcome per se. Instead, it becomes the starting point of a more transcendental spiritual desire. The unfulfilled bodily wants and desires of the first verse yield, by the final verse, to a yearning for union with God. The poet ends by saying that by withholding the fulfillment of his earthly desires, God keeps him from being half-hearted in his desire for union with the divine.
Notes:
- My Bengali is dreadful, so rely on the translations above AYOR
- The romanization of Bengali uses the iTrans scheme.