Love accounts for the remaining 20% of Mood. A Genopet with maximum Love will have 20% of their maximum possible Mood without taking other needs into account.
Petting is the player action which can increase Love. Love decays every 108 hours and will decay to 0 after 216 hours, if unaddressed. When Love is 0, a Genopet's Mood is effectively capped at 80% of its possible maximum.