It's just a Japanese culture thing. They're a very "no touch" society, so PDA is rude/scandalous. It's also why they bow to show respect rather than handshake. The longer and lower the bow, the more respect they're conveying to the person they're bowing to.
I didn't know about the PDA thing. Though it's their culture... though it wouldn't make much of a difference to me, either way I'm self-concious in public anyway xDD; *afraid to look like an idiot*