Analysis:
This quatrain by Saadi Shirazi delves into the complexities of human nature, particularly the allure of beauty and the role of desire in our actions.

Line 1: کس عیب نظر باختن ما نکند

"No one can fault us for losing our sight"
Here, "losing our sight" is a metaphor for being blinded by love or desire. The speaker is justifying their infatuation, suggesting that it's a common human experience and not something to be ashamed of.

Line 2: زیرا که نظر داعی تنها نکند

"For it is not only our desire that leads us astray"
The speaker is arguing that desire alone is not to blame for their actions. There are other factors, perhaps innate tendencies or external influences, that contribute to their infatuation.

Line 3: بیکار بهیمه‌ای و کژطبع کسی

"There is an idle beast, a person of crooked nature"
This line introduces a more cynical perspective. The speaker is suggesting that there is a base, animalistic part of human nature that is drawn to beauty, regardless of moral or ethical considerations.

Line 4: کو فرق میان زشت و زیبا نکند

"Who does not distinguish between the ugly and the beautiful"
This line further emphasizes the idea of a base instinct that is drawn to beauty without discernment.

Deeper Meaning:
This quatrain explores the tension between human reason and desire. The speaker is grappling with the idea that while they may be aware of the potential consequences of their infatuation, they are nonetheless drawn to the object of their desire. It suggests that sometimes, our desires can overpower our better judgment.

Key themes explored in this quatrain include:

The power of desire: The speaker acknowledges the overwhelming force of their desire.
Human nature: The quatrain suggests that humans are inherently flawed and prone to making mistakes.
The role of beauty: Beauty is portrayed as a powerful force that can cloud judgment.

This quatrain offers a nuanced perspective on the human condition, exploring the complexities of love, desire, and the struggle between reason and passion.