One of the main reasons why people stop being objective is because of emotional involvement. No, I'm not saying that you have to detach yourself from how you feel about the subject matter. I mean this in the sense that people sometimes get so caught up in how strongly they feel about an issue that they fail to see the subject as a whole. There's actually a psychological term for this sort of phenomenon: cognitive inflexibility. Have you ever met someone that refused to listen to your side of the argument? (Even if they're clearly wrong?!)
That's what I'm getting at. You don't want to be that person. Remaining objective is how you become a reasonable and open-minded individual. More importantly, if you have a point that you wish to defend, this is how you can present that argument without having people write you off as a stubborn ass after speaking for 22 seconds.