Analysis:
This quatrain by Saadi Shirazi is a beautiful expression of deep love and devotion. The speaker is expressing a profound longing for their beloved and a willingness to sacrifice everything for them.

Line 1: شب نیست که چشمم آرزومند تو نیست

"There is no night that my eyes do not long for you,"
The speaker's eyes, symbolizing their heart and soul, long for the beloved every night, indicating a constant and intense yearning.

Line 2: وین جان به لب رسیده در بند تو نیست

"And this life, near death, is not free from your captivity."
This line suggests that the speaker's life is entirely consumed by their love for the beloved, and they feel as if they are on the brink of death due to this intense longing.

Line 3: گر تو دگری به جای من بگزینی

"If you choose another in my place,"
The speaker acknowledges the possibility of being replaced, but they express their unwavering loyalty.

Line 4: من عهد تو نشکنم که مانند تو نیست

"I will not break my promise to you, for there is no one like you."
The speaker reaffirms their commitment to the beloved, emphasizing the beloved's unique qualities and the impossibility of finding a replacement.

Themes explored in the quatrain:
Unconditional love: The speaker's love is unconditional and unwavering, regardless of the circumstances.
Longing and desire: The speaker's yearning for the beloved is intense and consuming.
Sacrifice: The speaker is willing to sacrifice their own happiness for the sake of their beloved.
Uniqueness of the beloved: The beloved is portrayed as unique and irreplaceable.

This quatrain is a powerful expression of romantic love, characterized by deep devotion, longing, and sacrifice. The speaker's willingness to endure pain and suffering for the sake of their beloved highlights the intensity and all-consuming nature of their emotions.