Excerpt from the Feynman Lectures, Volume III, Quantum Behavior:
At those places where the two waves arrive at the detector with a phase difference of $\pi$ (where they are “out of phase”) the resulting wave motion at the detector will be the difference of the two amplitudes. The waves “interfere destructively,” and we get a low value for the wave intensity.
I watched Crash Course Physics #17 to visually understood what it means for two waves to "interfere destructively". That was lovely. But Feynman suggests that the out of phase condition is reached only when the phase difference is $\pi$, specifically. Why is that?