When the infusion lasts too long, more is extracted than the tea has to offer. As with water that is too hot, the compounds responsible for bitterness and astringency become dominant.
The tea then becomes harsher, drier on the palate, and the more delicate aromas recede into the background. It's not so much a question of "too strong," but rather of imbalance. What should have been subtle becomes dominant, and the tea loses its precision.
Conversely, a controlled infusion allows this balance between sweetness, depth and liveliness to be maintained, without one element taking over the others.
