The Behaviourist approach explains phobias as being a result of conditioning in relation to a phobic stimulus. According to Behaviourists such as Pavlov, most stimuli are initially neutral (we are not biologically determined to react to them). However, should we experience an unpleasant sensation in the presence of this neutral stimulus (e.g. the sting of a bee), we will form an association between the unpleasant sensation (unconditioned stimulus--the sting) and the neutral stimulus. This means that we then become conditioned to produce fear in the presence of the now conditioned stimulus (in this instance, the bee)--it has become an unconditioned response.
Skinner theorised that the maintenance of a phobia is down to operant conditioning. To avoid facing the phobic stimulus and therefore fear is rewarding, thus encouraging us to keep on avoiding what scares us. Similarly, to face the stimulus and therefore anxiety is punishing, so we are discouraged from confronting our phobia. This pattern of conditioned behaviour is unlikely to allow us to confront our phobia, and thus it maintains the unhealthy behaviour.