Human emotions have a long evolutionary purpose for our survival as a species. They are either a reaction to an external stimulus, or a spontaneous expression of an internal thought process. Emotions like fear are often a reaction to an external stimulus, such as when we cross a busy road the fear of getting run-over causes our evolutionary survival mechanism to take effect. These are external causes that trigger the emotions inside our brain.
However, emotions can be invoked as the result of an internal thought process. For example, If I managed to find a solution to a complicated mathematical differential equation, that could make me happy as a result of a feeling of personal satisfaction. It may be a purely introspective action with no external cause, but solving it still triggers emotions.