"Friends...?" No one's ever just asked Shizuo to be friends before. In fact, he doesn't even know how to react, and just ends up staring at Emmett blankly. He barely has friends as it is. Shizuo's not proud of his violent outbursts (though the ones involving Izaya are definitely justified), which apparently this man approves of, but still. Friends. He's so used to being feared and hated that this is really a shock to him.