User blog comment:Commander Marko/The advantage of Duo Queue/@comment-5556439-20130921175102

Biggest downside to Duo-Que is the threat of MMR inflation. Its fairly easy to win games when you first start doing ranked because 9/10 times your MMR will be a tad lower than where your supposed to be(Assuming you have been playing for a while so you should be at least slightly above average)

So the fact that your slightly lower than you should be, and you have someone else to communicate with makes it very very easy to inflate your rating and start to get some loses. Thats why I only Duo with people during series, it makes grinding in w/e division im in much easier since there's no possibility of a "hidden threat" so to speak.