Stefano and Kate go way back. She worked for Stefano as a prostitute when she was younger. In 2008, Stefano agreed to help get rid of all the evidence that proves Kate tried to murder Chloe Lane if she would marry him. Kate reluctantly agreed, and married Stefano. At first, Kate was clearly forced into it, but as time went on, she seemed to genuinely fall in love with Stefano.
So much so that when Stefano gave her the chance to leave him after he found out she was indirectly responsible for Sami shooting EJ in the head, she tried to reconcile with him. The relationship ended permanently in 2012 after Kate cheated on Stefano with Ian McAllister, and Stefano stole Kate's company as payback. Kate even used Stefano's later attempt at reconciliation to steal DiMera Enterprise from him.