soldier of fortune

Tonight, a busker decided to perform something not in his setlist. He looked straight at me and said he wanted to sing a song for "someone sitting over there".

And the song went -

"Now I feel I'm growing older

And the songs that I've sung

Echo in the distance

Like the sound

Of a windmill going round

Guess I'll always be

A soldier of fortune"

How did he know, that I am a wanderer longing for rest? Maybe the dust in my eyes gave it away.