Social learning theories work on the principle that our experience—be it positive or negative—such as phobia of lizards/cockroaches are the result of learning process which start early in life. Small children can play with snakes, they sire not aware of the danger involved. For them it is just another play object, as they grow up the fear of these things are instilled by their parents and society which is reinforced and accounts for reactions like phobia.
A psychoanalytical account for the same could involve attribution to some unconscious or/and repressed experiences. For example, suppose in your childhood you watched a group of roudy boys brutally torturing a cockroach/snake, which eventually died, although you going about the incidence after some days, but it might remain in back of your mind forever, which might explain your phobia to cockroaches which might remind you of the incidence and disturbs you emotionally.
