we all sneeze every now and then. a random one here and there is simply a momentary bodily response or reaction to some stimulus. but a bout of frequent sneezes is symptom either of a short term illness like a cold or of an allergy - still the body's reaction against external stimuli - and now you might need something to help alleviate the discomfort or some medication. take it further still, someone with a chronic sneezing problem, possibly sinus issues or something else within the body that causes it; this now you can be surer needs more medical help and its cause is within rather than without. 

replace the idea of a 'sneeze' above with any human emotion, especially an extreme one, and the concept of mental illness starts to become a lil clearer.