Analysis:
This quatrain from Saadi Shirazi offers a beautiful metaphor for friendship and its unique qualities. Let's break down each line:

Line 1: با دوست به گرمابه درم خلوت بود

"With a friend, I found solitude in the bathhouse,"
The bathhouse, a place of relaxation and intimacy, symbolizes the depth of friendship. The speaker finds solitude, not loneliness, in the company of their friend.

Line 2: وانروی گلینش گل حمام آلود

"Its flower-strewn floor was stained with bathwater."
The image of a flower-strewn floor stained by bathwater is a metaphor for the imperfection of beauty. It suggests that even the most beautiful things can be tainted by the realities of life.

Line 3: گفتا دگر این روی کسی دارد دوست؟

"He asked, 'Does anyone else have such a friend?'"
The friend expresses wonder at the depth of their friendship, questioning whether anyone else could have such a special bond.

Line 4: گفتم به گل آفتاب نتوان اندود

"I replied, 'One cannot coat a flower with sunlight.'"
The speaker's reply is a poetic way of saying that their friendship is unique and cannot be replicated. Just as a flower cannot be artificially given sunlight, their friendship is a natural and beautiful thing that cannot be forced or imitated.

Deeper Meaning:

This quatrain celebrates the beauty and uniqueness of true friendship. It suggests that:

Friendship is a sanctuary: The bathhouse is a symbol of refuge and intimacy, representing the safe space that friendship provides.
Imperfection is a part of beauty: The stained flowers symbolize the idea that even in the most beautiful relationships, there may be challenges or imperfections.
True friendship is rare: The friend's question implies that such deep connections are not easily found.
Friendship is a gift: The final line suggests that true friendship is a gift that cannot be replicated or manufactured.

This quatrain beautifully captures the essence of friendship, highlighting its depth, uniqueness, and the joy it brings to our lives.