[Mead] objected to Watson's casting the individual in terms of a physiological behaving animal without the intervening variable of reflective thought. Reacting [to] behaviorism, George H. Mead declared that his own psychology was a "social behaviorism." Mead's social behaviorism involved a conception of human nature very different from that held by other behaviorists. (Karier, The Individual, Society, and Education)
The principle objection Mead had against behaviorism as conceived of by other behaviorists--and it was a very significant objection--was to their definition of human nature purely in terms of physiological instinct and reaction. Behaviorists like Watson and Skinner asserted that humans are like animals in nature. Mead asserted that humans differ from animals because of our capacity for "reflective thought," the cognitive ability to think and reason based on evidence and persuasion that may lead us to decisions and actions that oppose instinctual or physiological responses.
Mead differed from other behaviorists (as he did identify himself as a behaviorist) by asserting a social behaviorism predicated upon the distinctive human characteristics meaning, language, mind (cognition), selfhood and rational reasoning, or thinking. Mead asserted the social nature of language and self identity thus constructing his objections to behaviorism.