Sosuke is a stupid asshole. Not only does he spy on Saki for no reason except to make sure she doesn't even deserve to look at another guy (it's not even implied Sosuke is jealous. He just wants her to remain away from dating), he then makes a painfully strange jump that after just ONE date with a flutist and seeing her fixing her hair in front of a shop window that coincidentally had a wedding dress inside must mean she wishes to marry him. Plus, his actor does a really wretched job this episode. He overacts more than usual and doesn't deliver any of his lines convincingly. Then again, it's not like he's given much to work with.
That paragraph above contains the thoughts I had to write down while I was watching episode 6. I was so dumbstruck by how much of a dick Sosuke was that I couldn't just let those thoughts stick, I had to get them down as soon as I possibly could. Thing is, the future episodes continue to do him no favors.
