Analysis:

This quatrain by Saadi Shirazi is a subtle exploration of love, loyalty, and the nature of beauty. Let's break down each line:

Line 1: هرچند که هست عالم از خوبان پر

"Though the world is full of beauties,"
The speaker acknowledges the abundance of beautiful people in the world, representing a wide variety of origins and backgrounds.

Line 2: شیرازی و کازرونی و دشتی و لُر

"Shirazis, Kazerunis, Dashtis, and Lurs,"
Here, Saadi mentions specific regions of Persia (modern Iran), representing the diversity of the people and their beauty.

Line 3: مولای منست آن عربی‌زادهٔ حُر

"My master is that free-born Arab,"
Despite the abundance of beautiful people, the speaker declares their loyalty to a specific individual, an "Arab." The term "free-born" implies nobility or a distinguished lineage.

Line 4: کاخر به دهان حُلْو می‌گوید مُر

"Who calls bitter what is sweet at the end."
This line is the most intriguing and open to interpretation. It suggests that the beloved, despite their apparent perfection, has a unique quality that sets them apart. They are capable of finding bitterness even in sweetness, perhaps implying a deep understanding of life's complexities or a certain level of discernment.

Deeper Meaning:

This quatrain can be interpreted in several ways:

The nature of love: The speaker's devotion to a specific individual, despite the availability of many other beautiful people, highlights the unique and personal nature of love.
The search for authenticity: The beloved's ability to find bitterness in sweetness suggests a search for authenticity and a refusal to be swayed by superficial beauty.
A spiritual interpretation: The "Arab" might symbolize a spiritual guide or a higher power, and the ability to find bitterness in sweetness could represent a deeper understanding of the nature of existence.

This quatrain is a celebration of individuality and the complexities of human relationships. It suggests that true beauty lies not just in physical appearance but also in the depth of one's character and understanding of the world.