Analysis:

This quatrain by Saadi Shirazi is a poignant expression of longing and the vast distance that separates the speaker from their beloved.

The Sweetness of the Beloved: The speaker's inability to imagine the sweetness of the beloved's mouth underscores the intensity of their longing and the pleasure they associate with the beloved's presence.
Social Class and Inaccessibility: The imagery of the king's palace and the beggar's tent highlights the social and physical distance between the speaker and the beloved. This suggests that their love is thwarted by external circumstances or societal barriers.
Unrequited Love: The final couplet implies a sense of unrequited love. The speaker cannot ascend to the level of the beloved, and the beloved does not descend to the level of the speaker.

Themes:

Longing and Separation: The central theme is the pain of separation and the intense longing for the beloved.
Social Class and Inequality: The quatrain hints at the barriers imposed by social class and the challenges faced by lovers from different social backgrounds.
Unrequited Love: The speaker's inability to bridge the gap between them and the beloved suggests the pain of unrequited love.

Deeper Meaning:

This quatrain is a beautiful expression of the human condition, exploring themes of love, loss, and the limitations imposed by social and personal circumstances. The speaker's longing for the beloved is a universal experience, and the imagery of the king's palace and the beggar's tent serves as a powerful metaphor for the barriers that can separate lovers.

This quatrain is a poignant exploration of the pain of separation and the longing for love.