Analysis:
This quatrain by Saadi Shirazi beautifully captures the pain of unrequited love, using the imagery of a natural phenomenon to express emotional turmoil.

Line 1: نوروز که سیل در کمر می‌گردد

"As spring comes and the floodwaters rise,"
This line sets the scene of spring, a season often associated with renewal and hope. However, in this context, the floodwaters symbolize the overwhelming emotions of the speaker.

Line 2: سنگ از سر کوهسار در می‌گردد

"Stones tumble down from the mountainside."
The image of stones tumbling down a mountainside adds to the sense of chaos and upheaval caused by the speaker's emotions.

Line 3: از چشمهٔ چشم ما برفت اینهمه سیل

"Such a flood has flowed from the spring of my eyes,"
Here, the speaker directly connects their emotional pain to the imagery of the flood. The "spring of my eyes" refers to their tears, suggesting that they have wept profusely over their unrequited love.

Line 4: گویی که دل تو سخت‌تر می‌گردد

"It seems your heart has grown harder."
The final line expresses the speaker's perception that their beloved's heart has become hardened, perhaps due to their unrequited feelings.

Themes explored in the quatrain:

Unrequited love: The central theme is the pain of unrequited love, expressed through the metaphor of a flood.
Nature as metaphor: The natural imagery of the flood and the mountainside helps to convey the intensity and overwhelming nature of the speaker's emotions.
Hopelessness: The suggestion that the beloved's heart has hardened implies that there is little hope for the speaker's love to be reciprocated.

This quatrain is a poignant expression of heartbreak. By comparing their emotional turmoil to a natural disaster, Saadi emphasizes the overwhelming and destructive nature of unrequited love.