Between 15 and 24 months, children take big steps in self-awareness and become more aware of themselves as individuals. They also become more aware of what they want and what other people want. At this stage children are learning that other people have beliefs, desires, and feelings that differ from their own. When they become more aware of their own desires and the desires of other people, children begin to show envy
Lewis, M. (2000). The emergence of human emotions. In Lewis, M., & Haviland-Jones, J. M. (Eds.), Handbook of emotions (2nd ed.). New York: Guilford Press.