pain in love ... crushes, arrows, etc.
"Falling" in love is the original metaphor, Wordwind, so perhaps "crush" takes its clue from this.

A "crush" is the junior version of a full-blown passion. Love blows us over, like a force of nature, while a "crush" merely weakens our knees.

Personally, I cannot believe that people smitten by a "crush" or Cupid's arrow experience any pain at all, de troy's observations notwithstanding. The arrow metaphor simply describes the suddenness and force of this uninvited, unexpected, emotion. Likewise "crush". It describes the nature of the impact, not the impact itself.

In the rush of a "crush", there is no thought of the pain which may follow.