"All of his poetry is about love."
Translation:Onun bütün şiirleri aşk hakkında.
The 'onun' is included in the ending of şiirlerİ and - by the way - 'onun' is not the subject of this sentence. So why could I not leave out 'onun'? The only reason I could imagine would be that şiirlerİ can be translated as her/hıs or theır poems, means to avoid ambiguity.