One of the hardest things to learn in League of Legends is what an opportunity looks like. Sometimes players will jump in at what seems like arcane times because they recognized an opening in the enemy’s positioning or cooldowns. This sort of awareness can only come from consistent play, as it arrives from knowing what each champion is capable of at each point of the game.
Unlike many positions in League of Legends, support can often be primarily a reactive position, waiting on key signs from the enemy or friendly champions for ‘optimal timing.’ Simply put, it’s easier to know to use your shield when an enemy is attacking than it is to know when to jump in attacking.
This is also true for less defensive champions. Aggressive supports like Thresh and Leona are great for having VERY clear timing for their abilities. Did you hit your opponent with your first ability? No? Then you don’t need to commit.