Why do people flip out over small PDA?

For example, if someone gives their boyfriend or girlfriend a kiss goodbye some people will say, "Eww! Get a room!" I don't see what's wrong with showing the world that you like/love somebody. One time when I was still dating my ex we were walking with our arms around each other. My friend Billy goes, "Get a room!" I don't know if he was just joking around because he jokes a lot but even friends put their arms around each other.