This question has been asked of me, many times and in many different ways. My answer remains the same. However, let me put a different spin on it this time.
People will seek therapy for a wide variety of reasons.
They seek therapy when their problems become unbearable or when they are otherwise motivated to seek help. I would recommend that someone seeks therapy when they experience stressful life circumstances which start to impact his or her ability to function normally.
Some stressful life scenarios can be
Seeking change is by no means an easy task. Sometimes a person's view of the world and his or her self image are distorted because of pathological early relationships that have been reinforced by years of negative life experiences.
Therapy and/or counseling can help people learn more adaptive ways of perceiving, evaluating, and behaving in spite of these negative life experiences.
A person should never be forced to seek therapy as in a situation where the client is non-compliant, the outcome may not be beneficial.
However, when one is motivated to alter their present intolerable mental state, then do seek professional help. Please don't wait until your mental state becomes so out of control that the integrity of your very being, is compromised.
Take the first, seek therapy!