LP dropping while I am winning

I've recently realized that the amount of LP that I get drops dramatically even when I win games. I'm currently on Gold 2, 1 (depending on when you see this), and I have 53% win rate out of my total ranked games. But I gain about 14~15 LP even when I am on about 5 or 6 winning streaks. I have a duo that is Silver 3/2 that I always duo with, is that the reason for my LP? Like does playing with lower ranked people drop your MMR?

I don't think it's accurate to say that your LP is dropping while you're winning. You gain LP when you win, even if it's less than what you feel that you deserve.

How do MMR & LP work together?

Think of our Match Making Rating (MMR) as a "GPA" that we have in League, much like you might have a GPA in the American school system.

As we score higher on tests, our GPA will improve. Given that all of our tests are weighted equally, we can change our GPA dramatically with one test early in the semester. At the end of the semester, we've already taken many tests and have essentially "earned' our GPA.

Like our GPA, MMR is a composite score -- it reflects our performance in our past games (or tests, in this example). The more games we play, the more data Riot has to gauge our skill level and the more certain it can be that our MMR is accurate. For that reason, it's harder to change our MMR after playing many games.

What if I play with my friend?

In order to match you with players of similar aptitude, Riot needs to guess how well you and your friend play together as one unit.

Let's say MMR is calculated out of 100. You have an MMR of 90 and your friend has an MMR of 80. For the sake of simplicity, let's assume that you're likely to win the game for your team just as much as your friend is likely to lose the game for your team. Riot splits the difference and gives you a joint MMR of 85. You'll end up playing with opponents who are at the 85 MMR level when you play together.

Playing with a friend who is rated lower than you will lower your joint MMR -- that's the point! You should play with people who are at your skill level.

If you continue to win together, you'll raise your individual MMR in the process.

Caveats

This is a really simplified way of looking at MMR and LP. Riot hasn't fully explained how the rating system works but we do know that recent matches are rated more heavily as it's possible to change your rank. Again, they may not give your recent wins as much weight as you'd like, but you can slowly but surely changer your MMR if you continue to win. Good luck!