I mean, we didn't need science to know that, but it's nice to have confirmation. It's been proven before that arguing and debating can also be addictive.
My pet theory is it's exacerbated by the "like" button phenomena. Most people are naturally driven to pettiness and will generally take any opportunity to exert control over others, however insignificant, much like mods on Reddit going power crazy the way they do. They will search for and take pleasure in anything that confirms their bias. The mere presence of, or the the ability to upvote and downvote things, gives people an incentive to attempt to "win" the argument. Just look at Twitter, for example. If you get more comments than likes, people will relish the "ratio" because of the implication.
On Reddit, people use the downvote button as a disagree button, even though that's not what it's for. As a matter of fact, there is objectively nothing that downvoting accomplishes that would not also be accomplished by simply leaving undesired content at a neutral score. Literally nothing would change, because highly downvoted comments are still seen regardless and upvoted posts would still supersede neutral posts.