Most people are not trained in how to express themselves appropriately, how to listen attentively, and when or how to respond compassionately or effectively. Even those who are trained don’t always practice what they’ve learned or don’t always practice these ways of communicating very well. This is especially so when one or both (or more) parties are under pressure.
Furthermore, family and friends usually have some investment in what you have to say, and you usually have some investment in them seeing you in a certain way. You may find yourself excluding important aspects of yourself to protect the relationship or an image. Also, with friends and family, you may be vulnerable to their influence and unclear about what are their thoughts and feelings versus your own.
Relying on friends and family to identify and to resolve your problems in a growthful way sets you and them up for disappointment (what happens when you want to return to a relationship that your friend helped you to leave, or when you hate the job your friend advised you to take?). It can also create a dynamic where one person feels superior or more healthy than (rather than equal to) the other. A relationship of this design develops a particular need to maintain status quo. In other words, at some level, both parties may work against true progress, healing, and change in order to preserve the relationship itself.
Significantly, unlike friends and family, therapists are legally bound to hold everything you say in complete confidence. (The exceptions to this are child abuse or serious threats to hurt yourself or some else.) Again, this makes the therapeutic setting profoundly private.
Therapy is a uniquely safe place to find your own voice and your own answers.