Read the excerpt below and answer the question. Outside the Eastern Gate

Outside the Eastern gate

Are girls many as the clouds;

But though they are many as clouds

There is none on whom my heart dwells.

White jacket and grey scarf

Alone could cure my woe.

Beyond the Gate Tower

Are girls lovely as rush-wool;

But though they are lovely as rush-wool

There is none with whom my heart bides.

White jacket and madder skirt

Alone could bring me joy.

Based on this excerpt from Book of Songs, what is the theme of this poem?