The phrase "thunder only happens when it's raining" is a metaphorical line from the song "Dreams" by Fleetwood Mac. Literally, thunder typically accompanies rainstorms, but metaphorically it suggests that some events or emotions only happen under certain conditions. In the context of the song, this phrase implies that certain troubles or relationship issues ("thunder") arise only when accompanying circumstances ("rain") are present. It captures how certain difficulties are natural and tied to specific situations, like how thunder naturally occurs with rain. The line also suggests an inevitability or a signal that something is happening—when the "thunder" is heard, the "rain" is present, meaning the underlying problem or emotional turmoil exists.

Scientifically, thunder happens as a result of lightning, and lightning can occur without rain, so the saying is more poetic than literal. But as a metaphor, it effectively conveys how certain conditions must coexist for some events to occur, especially in relationships as described in the song.