Soft and loud. Old and neon. Sad lyrics over a happy beat. Opposites rubbing together make sparks.

Contrast is what keeps attention awake. Two things that agree produce a smooth surface the eye slides off, while two that argue force a viewer to hold both at once, and that holding is the feeling people call interesting. The tension does the work, not either ingredient on its own.

Look at something you are making and find where every part is pulling the same direction. Introduce one element that does not belong by the logic of the rest, then see whether the whole thing wakes up or falls apart.