While it's not always the case, more often than not when someone says, "That's your opinion," what they're really saying is, "I don't like what I'm hearing, but I can't disagree with you without sounding either selfish or absurd." Because they base their convictions on preferences more so than principles, they can't champion their perspective directly. So they avoid having to answer for the pain and problems that occur when their perspective is put into practice by manipulating the conversation in a way where you can't dispute their position without sounding as though you're denying them the right to have an opinion.