As someone who also has been playing for 5 years (Mordekaiser was the new champion release when I started), the fun of the game for me was because nobody knew what they were doing and the game lacked structure.
**There was immense reward for those who picked up the game well and knew how to play it right, and there was very low penalty for those who struggled to play the game right. **
Now look at League's culture. People are paying actual money to take classes from pro players. Playing the game has become a job for the average player, even if they aren't being paid for it. Everybody knows what they are doing now, even if they may not have the mechanics to perform those actions correctly. Now that there is a set meta-game, a structured way to play, there is more room for frustration for this team game because of team-based issues.
For example, post-beta League was a lot like basketball; everybody playing can be involved and doing something, but there is a lot of room for one player to carry really hard, and it's not that big of a deal to the newbies because that player is just really good at the game. The experienced player can do the same actions that every one of his teammates can, at any time of the game.
Current League is a lot like baseball; everybody has their own role and are limited to that one role, they cannot fill in for others at any point in the game, so any deficiency in the team's play can be pinpointed on a particular player not doing their job, and it is so obvious who is holding a team down given the structure of the game. Your first base man cannot also be fielding. Your batter pitcher cannot also be catcher. You can have great players guarding the bases, but if your fielders are shit, your whole team is going to be shit at defending.
And this is why current League is more prone to toxicity and negativity. It's easier to flame or get angry about players for not doing what they need to do, because the structure of the game defines that player's job, and everyone in the game knows it. All team's have a similar structure, so it's easier to compare/contrast players of the same role on the opposing teams. It's easy to rage over the fact that your left fielder can't catch/throw a ball to stop the other team from running the bases, while the other team's left fielder catches every ball and your team can't score at all because of it.
I'll even admit that my frustrations with the game come from this dynamic. It's when my teammates are completely outclassed by their counterparts or refuse to cooperate with the rest of the team. That one player who never follows their roaming lane opponent. That one player that calls Missing after their laner has already killed you. That one tank/support who is split pushing instead of team fighting. That one jungler who is always second to show up for ganks, only responding to the enemy jungler's movements. That one Marksman who refuses to auto attack in a team fight when only threatened by a zero-damage, cooldown-blown tank, and says "I NEED PEEL FROM LEONA". The structure of the game labels those who are weak and those who are strong, because of the criteria it provides to measure players by.
Now I'm not saying game structure is bad, but it certainly contributes to toxicity and negativity when the masses play the game by a certain structure, and it re-defines the conditions by which you have fun. You can have fun in basketball regardless of how everyone else is doing. You can't have fun in baseball when no one is working together.
Current League is baseball, and ARAM is basketball.