You probably behave differently when you are having a bad day than during a great one. For example, while you might politely smile at a neighbor you dislike on an otherwise pleasant Saturday afternoon, that same interaction will elicit a scowl when you are rushing off to work Monday morning. In other words, your brain takes recent experiences into account when coordinating your responses to external stimuli. Even simple animals, like the fruit fly Drosophila melanogaster, do this: Sometimes, a male fly will display high levels of aggression toward another male, while at other times, two male flies can coexist peacefully. All Drosophila brains are wired the same way, so why do the flies exhibit different behaviors when presented with the same sensory cue?
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Source: Phys.org