Analysis:
This quatrain by Saadi Shirazi delves into the complexities of human relationships, particularly friendship. It suggests that maintaining friendships requires patience, understanding, and sometimes, the ability to endure hardship.

Line 1: با دوست چنانکه اوست می‌باید داشت

"One must treat a friend as they are,"
This line emphasizes the importance of accepting friends for who they are, including their flaws. It suggests a kind of unconditional acceptance.

Line 2: خونابه درون پوست می‌باید داشت

"And endure pain, hidden beneath the skin."
Here, Saadi uses a vivid metaphor to convey the idea of enduring hidden pain or suffering for the sake of friendship. It suggests that maintaining friendships can sometimes be painful, but it's a pain worth enduring.

Line 3: دشمن که نمی‌توانمش دید به چشم

"Since I cannot bear to see the enemy,"
This line contrasts the idea of friendship with enmity. The speaker finds the idea of having an enemy so distasteful that they would rather endure the pain of a flawed friendship.

Line 4: از بهر دل تو دوست می‌باید داشت

"For your sake, I must keep a friend."
The final line suggests that the speaker is maintaining the friendship for the sake of someone else, perhaps a lover or a higher power. It implies that there is a deeper reason for their endurance.

Themes explored in the quatrain:
The nature of friendship: Saadi suggests that friendship is complex and often requires sacrifice.
Tolerance and acceptance: The speaker emphasizes the importance of accepting others for who they are, even when it's difficult.
The cost of friendship: Maintaining friendships can sometimes be painful, but the speaker believes that the benefits outweigh the costs.

This quatrain offers a nuanced perspective on friendship, suggesting that it is a valuable but often challenging relationship. The speaker's willingness to endure pain for the sake of their friend highlights the depth of their commitment.