From a child to an adult, in the process of growing up, we are always defining ourselves, trying to answer the question about who we are. Although there are many ways through which we can find the answers, it is primarily through our identification with social groups that we define ourselves.
In a society, we are all divided into different social groups -- White, Black, Asians, Africans etc, each of which has a social identity that members share together. These identities are the basis for most people to evaluate themselves. For instance, we can often hear people saying "We Americans are optimistic people" or "As a student, I'm not supposed to do things like that". This kind of self-definitions are based on characteristics of the social groups we come from. Whether suitable or not, these identifications are tend to be accepted in the bottom of our heart, since it is by such definitions that we divide the world into "them" and "us" and therefore distinguish ourselves from "the others". People's enthusiasm for the Olympic Games is a good example. When an athlete from our own country gets a medal, we would feel proud and confident. Although as audiences, it is not us who win the game, that medal raises the status of our social group as a whole, thus our self-evaluations are improved.
Besides shared social identities, we can also define ourselves through interaction with other members in the social group. Compared to comments from strangers, we often care more about evaluations from people who are familiar to us. A bad comment from a stranger might be forgotten in a second, while our self-definition would probably be effected greatly if a friend says something bad about us. Additionally, we are likely to learn more about our strengths and weakness in the process of taking part in different activities with members of the same social group. Rowan Atkinson, the actor of Mr. Bean, first detected his acting talent in a theater club in college. While acting on school festival with other members, he started to love acting and decided to become a professional actor. Similar situations are taking place in our everyday life as well. By joining in clubs and school unions in universities, undergraduate students get to know their strengths and start to build their identifications.
However, some people may argue that things such as psychology tests or self-criticism are also effective ways to define ourselves. Indeed, we cannot neglect those self-evaluating methods, but they are not that widely accepted as self-evaluation through social groups. Take psychology tests for example. Although popular on the internet, most of such tests do not have much scientific basis, and therefore are not taken seriously by most people. On the other hand, other professional tests offered by psychologists or psychological organizations are often too expensive for common people. As for self-criticism, it's hard to say that the standards for such criticism are not greatly influenced by shared identification with social groups or comments from group members.
In a nutshell, social groups are constantly effecting our self-definition. The identifications of social groups and the comments from group members form a major part of our self-identification. It is primarily through the eye of others that we get to find the answer about who we are.