What do you think leads to fundamental change? How or why does therapy work for some people and not for others? What do great therapists do that get people to change?

It is important to understand that mental disorders are just as serious as any other physical disease of the body. The treatment, however, is slightly different. While there are instances when the combination of medications and therapy gives the best results and has the best outcome for the patient, it is notable to mention that the main practice for treating mental disorders is usually conversational therapy with a licensed psychiatrist or psychologist. In some cases, therapy may work like a charm, and the patient may feel better after their sessions. However, for some of those who struggle with a mental disorder, no matter its severity, therapy might not work at all.
One of the main reasons that this happens is because the patient expects results immediately; some patients think that therapy is something that works as fast as a pill. When they realize that they're not immediately feeling better or more at ease, they become sadder, more depressed and angry, and try to find someone to blame, be it themselves or their therapist. Often, they will give up.
I believe that it is of utmost importance for patients to realize that it takes time to heal the inner turmoil and emotional pain one might feel. Just like you wouldn't expect to learn a song the first time you hear it, you also shouldn't expect your mental struggles to magically disappear the first time you talk with your therapist. No matter what your therapist says to you, your situation will not fix itself over night.
Furthermore, it is important for patients to realize that therapy is just one part of the treatment. If we want to be better, we need to listen to our therapist's advice and make some changes in our lifestyle. The suggestions might vary depending on the type and the severity of the mental disorder, but usually therapists recommend a healthy and balanced diet, less procrastination, more physical activity, a clean and healthy environment, and most importantly, a regular schedule. A good therapist will try to be as helpful and as accommodating as possible; a great therapist will explain to patients that getting better is a process that requires teamwork. They will be able to provide patients with the confidence to make these necessary fundamental changes, which will drastically improve the patient's prognosis.
Finally, therapy might not work if the patient and the therapist simply do not match. Their personalities or their views and opinions might be so different that they clash and cancel each other out; this will not help the patient at all. Therapists mustn't judge their patients under any circumstances, whatsoever; on the contrary, they should try to understand their patients, accept them as who they are, and make sure that their patients feel safe and comfortable with them. The patients should also try to work with their therapists; they must be patient, cooperative, respectful and honest. A good patient–therapist relationship will give good results.