The compound eyes of a fly grant it the ability to see the world in front of it split into possibilities. For this reason, it is difficult to swat a fly with a hand that it can readily see, as it can then simply leap forward into a permutation of reality in which it survives. Though it witnesses its own death in several lenses of its eyes, by sacrificing those possible futures, the insect is able to prolong its own life.
If the hand comes from behind, however, the fly is far more likely to be crushed. Through careful aim and positioning, all possibility of escape can be eliminated.
Cupid uses this principle to his advantage. For most human beings, the hand is always coming from behind. With such a narrow, occluded sense of time, the path from birth to death seems entirely linear to their species. The space of possibility, visible to many of even the simplest lifeforms, is entirely invisible to them.
As he draws his bow, Cupid watches for moments of hesitation and indecision: strangers who are too nervous to address one another, propositions about to be turned down, and goodbyes that are happening slightly too quickly. His eyes have many facets, much like those of a fly; as he focuses on a target, he sees their manifold futures of successes and mistakes, of happiness and suffering, of joy and regret.
He chooses his targets from among the gallery of possibility that he sees, then puts an arrow through each of their hearts. When they die, so too do the futures they occupy. One by one, he removes the frightened, the uncertain, and the shrewd versions of his victims, leaving behind the brave, the brash, and the impulsive. By the time he’s finished his job, only one possible future remains- romance curated through death.
In this sense, his arrows don’t actually cause mortals to fall in love. They are just as lethal as any others of their kind, but the deaths they cause are never seen. Those targeted go on with their lives hand in hand, blissfully unaware of what they lost along the way.