Effects of Duo Queue on MMR

Greetings,

I am a diamond 4 player that recently started playing duo queue a lot with a friend that is in platinum. To start, he was in plat 5 & I was in diamond 5. We won a high percentage of our games (estimate around 65-70%), and I'm now 100 LP into Diamond 4. However, I have seen a remarkable shift in my LP won per games, which tells me my MMR has gone down considerably respective of my rank. I went from gaining about 20 LP and losing 14 or so to gaining 12 LP and losing 36.

My question to riot is this: With a game that emphasizes team play and encourages playing with friends, why would a player's MMR take such a hit while playing within the confines of the system (diamond can only queue w/ plat, plat only w/ gold or diamond, etc).

Thank you, whipkey