Your Brain Perceives and Acts Upon Emotional Stimuli
Even though we think of emotions as internal states, psychologists define emotions as a combination of cognitions, feelings and actions [source: Kalat]. This means what we think of as "emotions" includes not only how we feel, but also how we process and respond to those feelings.
To understand this, it's helpful to consider the purpose of emotions. In 1872, Charles Darwin first published "The Expression of the Emotions in Man and Animals," which established that emotions serve an important evolutionary purpose. In order for a species to continue, it needs to survive and pass on its genetic information. Emotions like fear serve to protect you from danger so you can survive to pass on your genes. The "fight-or-flight" response that primes your body to defend itself or run away from danger is also an emotional reaction. Emotions like love and lust give you the desire to reproduce.
For these reasons, the brain takes on the function of evaluating a stimulus -- such as a dog that's about to attack or a beautiful woman batting her eyelashes -- and crafting an emotional response to it. The brain thinks in terms of how it can best respond to a situation in order to survive and reproduce, and it uses emotions as the catalyst to convince the rest of your body to act accordingly.