Stein is a singularly sadistic being. To him, everything and everyone in existence is just another experiment waiting to happen. He himself is not an exclusion to the rule - the scars on his body and the screw in his head are all experiments he's done on himself. He's constantly curious about everything around him, and anomalies interest him the most. To say he loves dissection is to grossly understate his obsession with taking things apart. Stein is driven by his curiosity and a tireless scientific drive, and dissection is simply his preferred method.