Here's why:

* Constructive interference happens when two waves combine to create a wave with a larger amplitude. This occurs when the crests of the two waves align (in phase) and their amplitudes add together.

* Destructive interference is what happens when two waves combine to create a wave with a smaller amplitude. This occurs when the crest of one wave aligns with the trough of another wave (out of phase) and their amplitudes partially cancel each other out.

Think of it like this:

* Constructive interference: Two people pushing a swing in the same direction, making the swing go higher.

* Destructive interference: Two people pushing a swing in opposite directions, making the swing barely move.