We can choose our emotions, but they also happen.
As humans, we have natural responses to occurrences depending on the severity of how it affects us individually. Although our hormones steer us toward one emotion as a response to an infliction of pain or discomfort, we can choose whether or not we linger in those mental states. One can choose to think on positive things to cheer themselves up. Emotions do automatically happen in a responsive being, but we can choose them when it comes to attempting to deal with them.
The short answer to this question is, in a way, both. We can choose our emotions, but at the same time, we have automatic responses to different situations that trigger different hormones.