I'll just post what the Rioter (Riot Moonleaf) actually said since it wasn't that they intentionally match you with worse players after a win streak. Copied straight from the link with minor formatting changes for clarity.
Win a lot -> MMR climbs -> more people have less MMR than you and less people have more MMR than you -> chance where you are predicted to be the strongest player in the team increases
Stats here (http://www.lolsummoners.com/stats/eune)
And on page 3
Quote: Harima
Last winning streak i had, long ago, after winning 5 in a row i got matched with the most ridiculous (yeah i was going to say re..ed) team
Statistically speaking, there is more or less a 20% chance that you have the most MMR (or tied for it) in your team. Additionally, if you're the worst player in the server there is a 0% chance you're the best in your team, and if you're the best player in the server there's a 100% chance you're the best in the team. Does that make sense in cold, hard numbers?
Quote: betsoni
bcs teams are matched against each other based on AVERAGE MMR and if u have higher mmr then rest of your team the enemy team gonna have better mmr then 4 other members of your team ( bcs they won't get sb like u with higher mmr then the other 4 but they all will probably be around same mmr ) so they gonna win all the other lane and this translate into no matter what u do u can't win the game
why riot do this ?
This is possible, but doesn't always happen. The matchmaking system is first trying to find 10 players with very similar MMR and number of wins. As seconds pass and it doesn't find 10 players fitting those restrictions, it will be less strict and broaden its search in order to find a game for you instead of having to wait for someone of your MMR to log in. If it's forced to team up players with different MMR, it will try its best to balance the two teams out.
Quote: Enjutsu
Btw what those stats are supposed to mean for your post? all it shows is player distribution among the leagues.
In EUNE, if you're Silver 5 you are better than 20.76% of the server. If you're Plat 5, you are better than 89.59% of the server, and if you're Challenger you're better than 99.98% of the server. Of course, a Challenger won't be matched with Bronze, but I'm trying to give you a perspective that as you climb up, there are less and less players that are better than you and can 'carry' you.
Quote: bÃ*nkai
And why can't we always play with people who are at our MMR? Games are way too one sided, they have almost always been, it's not enjoyable, you are either 16-2 at 15 minutes or 2-16 (team scores).
Because MMR is only a prediction of how someone will play. Someone who underperforms could have had a bad day or playing a champion/position he's not used to or be sleepy or on tilt. MMR can't predict human behaviour, only use past behaviour to make an educated guess.
Hope these help add more context on what I'm talking about. Of course, I'm not in the Leagues team, I'm just providing context on matchmaking statistics and formulas the way I understand them.
EDIT: I'll just drop this (http://en.wikipedia.org/wiki/Gambler%27s_fallacy) here. Discuss.
So people don't have to bother with the link (which they still can to check my transplant). Hopefully I made the formatting make sense, since it doesn't copy over too well.