If you look up your symptoms and they point you towards a certain illness, you may not be correct, but the symptoms have been there before. So even if the cause for your symptoms turns out to be different, I don't think behaviour in the patient will change as much.

Besides, even when the patien didn't try to selfdiagnose, their symptoms might point towards several illnesses. This is why in the beginning of therapy docs evaluate their patient, and their working theories may change over time.