Because you are increasing the likelihood of giving rise to drug resistant variants of the life-form those antibiotics are directed towards. There is a whole fi

Because you are increasing the likelihood of giving rise to drug resistant variants of the life-form those antibiotics are directed towards. There is a whole field of investigation into this phenomenon. A good (technical) starting point if you are interested in more is: http://www.drug-resistance.org/ http://www.drug-resistance.org/ Please, for the sake of society at large, complete any and all antibiotic/antiviral drug courses you are prescribed by your physician.