Olan'vora

From Memory Alpha, the free Star Trek reference

Olan'vora is a Varro biological chemical reaction. During and after mating, the epidermal skin will luminesce. Psychological bonding will increase with exposure, stimulating the areas of the brain responsible for love. With decreased exposure the effects will diminish in a matter of months. But, withdrawal symptoms can be unpleasant.

Harry Kim became romanticially involved with Derran Tal, a Varro female. She didn't believe it was possible for a Human to be affected by Olan'vora.

Tal said good-bye to Kim and left him with pain, since the more time they spent together, the harder it was to part. (VOY: "The Disease")