What do you think is the most likely reason for the development of drug resistance?

Drug resistance: The over prescribing of antibiotics.

See below: Unnecessary antibiotic use in animal husbandry. Unnecessary antibiotic use in treating viral infections. Unnecessary antibiotic use in hospitalized patients without bacterial infections. Bacteria must survive in a toxic competitive world and have developed adaptive techniques which include the ability to acquire resistance plasmids or mutations to survive exposure to toxic chemicals.