In the early 1970s, psychologists Altman and Taylor developed an important theory about the growth of friendships. They posited that mutual openness is the driving force behind relationship development. This means that when friendships develop, one person is open with the other, and the other responds openly.

Furthermore, it's essential that the intimacy of information shared increases in an equal way. Equality is essential. We perceive people who don't respond openly to our intimate openness as cold and distant. Moreover, we find it inappropriate for someone to share intimate information with us immediately upon first contact.

Researchers found strict rules regarding what information can be shared at what stage of a friendship. Violating these rules carries penalties such as negative judgment of the other person and sometimes social exclusion.

People with depression are often too open about their negative feelings. Their reaction is to tell them how to behave and think. If this isn't followed through quickly enough, social exclusion follows. Conversely, not being open, as is common among socially anxious people, also leads to social exclusion. Not being open is seen as cold and distant and sends the message that the friendship can't be developed further.

In psychotherapy, openness plays a key role. Being open about events you've personally experienced leads to psychological recovery.

See also Wikipedia: Social penetration theory