A psychologist is trained to identify certain traits while listening to a person talk, and knows how to translate body language as well. If a person wants to change, the psychologist can help that person explore the reasons he or she is doing whatever it is that's not-good, and may be able to suggest time-proven ways, or tools if you like, of dealing with things without using the not-good behavior. After all, behavior is learned, so in a person of typical intelligence and ability it can be unlearned. While realizing that personality is inborn, I think babies, as they grow, are taught responses to feelings by example. Hate towards those who are different from you doesn't just grow naturally but is taught, and, minus physiological factors such as Tourette Syndrome or ADHD, so is violence or anxiety or over-fearfullness. Sometimes realizing that your anger or other emotion is identified with something deep within your past helps you to
unlearn your reaction to it. And there's also the cathartic effect of letting it all out to someone who's not and never will be connected to you. I think most people have all the tools they need to contral their behaviors and reactions, but sometimes need a guide to help you identify those tools.