There are many reasons that your friends are not a replacement for therapy. Not only is putting that burden on your friends unfair to them – your friends are not your therapists, and no matter how much they swear up and down they want to be there for you and don’t mind, they are not equipped to be giving certain advice and oftentimes may be not only steering you down the wrong path or worsening your own mentality surrounding a situation but their own mental health may be negatively impacted by trying to act as a counselor-type role when they’re not equipped to do so.
A therapist is not only removed from the situation but they are trained on how to help you and take care of themselves in doing so. It’s an ethical way to be able to talk about intense things and to vent about your life on a regular and lengthy basis – with your friends, it is not appropriate and can cause serious damage both mentally and to those friendships.
Additionally, in a therapist-client relationship, you don’t need to worry about your therapist’s feelings. Obviously, they’re still a person, and you can’t disregard that, but they’re there to help you with your problems, and they have signed up (and are being paid) to help you with difficult things – you don’t have to worry about being a “downer”. You also don’t have to censor or self-edit your speech because you don’t want to hurt their feelings or out of concern of being judged. Therapists are trained to be understanding, and chances are they’ve heard 10000x worse.
With friends, you often might get advice for the short term (and therefore gain short-term relief and gratification), but with therapists, you can chart a path of plans for how to work towards long-term goals, and therefore making much more significant progress and a much more significant and positive impact in your life.