According to the Appraisal Theory of emotions, how we feel about a certain situation is determined by our Appraisal or evaluation of the event.
Let's say you went on a job interview and you believe that the interview went well - you gave good answers to the recruiter's questions, and you have all the qualifications needed for the job, then you will have a positive emotion about it. You might feel happy and excited. But if you perceive that it did not go well, then you will feel negatively about the event. You might feel sad, dejected, or helpless.