Emotional states, moods, are an important part of a nervous system. States such as fear, exhaustion, etc. allow the choices an animal makes to incorporate the sum of feedback gathered from their senses.
When ants are "upset" they start attacking everything new and unfamiliar. They will bite your finger, the tweezers, the water feeder, other insects (but never their sisters.)
When ants are calm they will explore these same objects, they will notice they can collect water from the feeder.