Premades are legit cancer half of the time. League has a problem with toxicity, and premades crank it up a notch because it nets your team another douche. If you aren't a toxic shit, odds are you won't duo with one, so when one of the two is toxic it's a safe bet that the other is too. The reverse is also true though chill league players are relatively rare.
Beyond that, premades are generally annoying to deal with due to how they constantly give off this impression of an "us vs them" mentality even in their own team. I've literally never seen a duo fault their partner, it's **always **someone else that'll have to put up with their finger pointing. Always trying to get their duo into their main role. Often neglecting other allies in favor of assisting their duo. Always trying to take the driver's wheel. I've met some really cool and chill duos, but I generally hate dealing with them. Bonus points if one is a jungler.