Mood is a feature of Tree World. The mood of a Critter is affected by the type of plant it is on. If it is on a plant that it likes, it will be Happy and produce more Tree Food. If it is on a plant that it doesn't like, it will become Unhappy and produce less Tree Food. If it is on a plant it's neutral to, the Critter will be content. You can see the Mood of a critter on its information screen or when you first place it on a branch.