(YAOI WARNING!) They always had a love hate relationship. Shizuo was always chasing Izaya with a street sign or a vending machine or anything else that was "on hand" while Izaya did anything he could do to piss Shizuo off. That was the way they did things. However, they had always wanted to be around each other....even in High School they were always secretly hpscanning the room for each other. Though they claimed to hate each other.... Is that really all it was?