I don't know that you can call anyone who has a nervous breakdown at 22 years old "mentally strong." It's all been discussed ad nauseam here in the past, but it has always seemed to me that drugs exacerbated issues that were already present within Brian. He'd always been pretty good at keeping them below the surface (using his music to express his inner turmoil), but as the drug experimentation got worse, everything was brought out into the open for the first time. Obviously there were other factors, but it isn't as black and white as "drugs made a happy and healthy Brian mentally ill."
I've always been a little bit confused as to what a nervous breakdown entails. I know Brian apparently had a panic attack on an airplane, but is that a nervous breakdown? Was there more to it, or was that it? (And I don't mean to minimize a panic attack. I've had them , and they are very scary.) But I do agree, Chris, that the seeds of mental instability were probably planted in Brian early on and just exacerbated by drugs. And then on top of it, he's a practical joker with an odd sense of humor.