Why are there Win streaks and lose Streaks???

Have you guys noticed how your match history is either entirely loses or entirely wins? Why does this happen! And the most annoying thing is that i end my win streak right before my promos! I would understand a coincident but for the last few times, i noticed my promos significantly harder than other games. I then lose the promos and fall all the way to 10 lp. Then for 4 games, its alternating win and lose, than suddenly win streak all the way to 100lp to repeat the cycle. If im lucky, i win 2 more games and get through promos but i lose 6 games in next division and fall back to previous divsion. Why riot??? I understand that tilt causes u to have streaks but i know it is clearly not tilt. I intentionally get matched with worst players as i get more wins in a row and vice versa. I thought it was that mmr changes in bigger quantities than lp does, but i noticed that my opponents skill level does not change. Its my own teams! How am i ever supposed to climb if riot is pulling me back?