Kuzehiko
That moment I found a lifetime treasure
That is what I recall from watching the show years ago. I might be wrong or not though. I am gonna rewatch it. However, official profile state "Raised to the limit" which is pretty much the same in my eyes. Ash and Greninja's bond raises to the limit thus synchronizing.When did the anime state that the transformation happens when the bond between Ash and Greninja reached maximum level? As I recall, the anime stated the transformation is possible because Ash and Greninja have a strong bond, but it happens when the two of them are completely synchronized. I don't recall the anime mentioning that their bond has to reach a certain level for that to happen. Their synchronization, yes, but not their bond. I don't think many people here would try to claim that Ash's bond with Greninja is stronger than his bond with Pikachu; and I didn't get that impression from the anime, especially not after it was explained that the Bond Phenomenon is tied to Greninja's ability, not just to its bond with Ash.
The fact that the Bond Phenomenon transformation happens so infrequently can be easily explained by the Bond Phenomenon trait/ability itself being incredibly rare. Then, once one finds a Pokémon with that incredibly rare trait, that Pokémon has to achieve an incredibly strong bond with its Trainer in order to activate the transformation.
And I said it's a suggestion to the opposite claim that Greninja and Ash's bond is not the strongest due to it have never been disproven.