Read the following poem by Li Bai: To Tan Ch’iu

My friend is lodging high in the Eastern Range,
Dearly loving the beauty of valleys and hills.
At Green Spring he lies in the empty woods;
And is still asleep when the sun shines on high.
A pine-tree wind dusts his sleeves and coat;
A pebbly stream cleans his heart and ears.
I envy you, who far from strife and talk
Are high-propped on a pillow of blue cloud.

Why does the poet envy his friend in the poem? Support your answer with evidence from the poem.