Yet [thinking there is presently a Utopian way of life] is both surprising and dangerous. Surprising because there has, perhaps, been no time in modern history when so large a number of human beings, in both the East and the West, have had their notions, and indeed their lives, so deeply altered, and in some cases violently upset, by fanatically held social and political doctrines. Dangerous, because when ideas are neglected by those who ought to attend to them - that is to say, those who have been trained to think critically about ideas - they [the ideas] sometimes acquire an unchecked momentum and an irresistible power over multitudes of men that may grow too violent to be affected by rational criticism. (“Two Concepts of Liberty,” in Four Essays on Liberty, Isaiah Berlin, 1969)
The idea of viewing individual human freedom as self-determination, self-realization or self-creation is a late historical development that stems from the ideas of philosophers like Rousseau, Hegel and Marx. Rousseau is particularly exemplary of this view of individual human freedom (Berlin used the term freedom as interchangeable and synonymous with the term liberty) as being self-determination, self-realization or self-creation.
Rousseau - Positive Freedom
Rousseau posited in his theory of freedom that individual freedom is achieved by participation in the general will, or social process, and that the general will paradoxically supersedes what Rousseau saw as the uniformed will of free individuals who require action by the government to "create the conditions necessary for individuals to be self-sufficient or to achieve self-realization" ("Positive and Negative Liberty," plato.stanford.edu)
Isaiah Berlin - Positive Freedom
Berlin's definition of positive freedom correlates to the concepts and ideologies espoused by those such as Rousseau, Marx and Hegel. Berlin found that the ideas subsumed under his concept of positive freedom were especially dangerous.
Berlin traced positive liberty back to theories of autonomy and self-rule. Berlin found Rousseau's theory of liberty particularly dangerous. ("Isaiah Berlin," plato.stanford.edu)
Berlin defined positive freedom (liberty) as the freedom and ability to pursue and achieve willed goals and also as the autonomy or self-rule which is opposite of dependence upon others ("Isaiah Berlin"). He asserted that this class of freedom was readily converted to perversion and considered it sinister. While he conceded in later years that he ought to have explained more of the dangers of negative freedom, e.g., Robber Barons of laissez-faire capitalism, he connected the evils of positive freedom with the development of fascism and communism.
Whereas modern Western thought equates positive freedom with liberty, equality, fraternity, the rallying cry of the French Revolution, Berlin found the premises of this Western ideology to be misguided and "dangerous."