The inability to hold a stable mental image of you when you're not present — so love evaporates the moment you're out of sight, distance reads as abandonment, and they may treat you as a stranger after a brief separation.
You traveled for a week. The first day they were warm. By day three they were cold. By day five you were a stranger; by day seven they'd half-decided the relationship was over. You came home to a different person. Five hours later they were warm again.