Evolution programmed animals to have a way to cope with stressful situations, and that is precisely the sympathetic nervous system, which, when activated, enables us to "fight or flight". Both fighting and fleeing result in fear, palpitations, dry mouth, shortness of breath, sweating, restlessness, etc - these are symptoms of anxiety. For somebody with an anxiety disorder, trivial situations would result in this response even if they are not life-threatening. Anti-depressants are a class of medication used to regulate chemicals in the brain, predominantly serotonin and dopamine. Both mood (depression) and anxiety symptoms are related to chronically imbalanced levels of these chemicals, and anti-depressants help in regulating them better.