I think that Sami feels guilty about keeping Grace from EJ and then finding out that Sydney was her’s feels she owes him something. I am not sure if this is a conscious feeling or sub-conscious.
As for EJ, again he shows his trus colors. He wants what is his(Sydney) and also wants revenge(Stefano, Sami and Nicole). He doesn’t think that he has done anythinkg worng, even though he threw Sydney out in the first place. If he hadn’t, he would have never lost her!!!