You just need someone in your corner...

There are two things which I think run in parallel with each other. These are art and madness and how they are viewed by others. Madness, for instance, is only defined by the majority; if one out of ten people, thinks they are sane, yet the other nine believe them to be mad - the majority rules, and they are mad. It is also the credibility factor; if that person is the Prime Minister, they have a much better chance of convincing people of their sanity than if they were a tramp on the street.
The same thing can be said of art. If a person makes a piece of work and calls it art, but nobody else agrees - is it still art? Yet if the person who made it, (no matter how ridiculous the work), was a respected known authority - would it be more accepted? Probably... Certainly... YES.
Therefore if you cannot prove something yourself, you just need someone on your side to back you up. In relation to art therapy for instance, this is something which is originally a diagnostic tool for doctors, but if put in a gallery - can be labelled as 'art'.

The stimulation of creativity lies in the indulgence of every thought. For example, when you are severely depressed, your brain is thinking: me, me ME! With such pandering of the self and disengagement of other needs or whims, how can one help but concentrate more on their own creative impulses?
This theory should be taken with a pinch of salt as it is simply that, a theory, not gospel.
To briefly reiterate what I was saying before; art-therapy has long been considered a useful and salubrious answer to those suffering from mental illnesses or an outside trauma of some kind. However the line between art-therapy and just plain art, is whether or not someone somewhere is backing you up. This might not happen until years after you die, and some inspirational young whippersnapper has come along and said 'Hey, that's not half bad!' It is artists such as Strindberg and Van Gogh who lend substance to the idea of suffering being a catalyst for their creativity. This stimulation of creativity has been reaffirmed by established artists. For instance, Van Gogh was reported to say once:

It is terrible when I suffer a crisis, I lose all concept of what goes on around me, but that tosses me right into work and causes me to take things seriously, the same as a lurking danger compels a miner to rush through his work.

The influence of suffering from mental illness on creativity can be viewed in several different ways. For example, it could be the cause or a cause of artists' great works; a catalyst or merely an enhancement. In a similar way that some consume alcohol before public speaking to improve their confidence.
I draw your attention to a collection of work which celebrates the idea of art-therapy as ART. This is the Prinzhorn Collection. In a nutshell, a man named Hanz Prinzhorn (1886-1933), art historian and doctor; built up a fascinating and unique collection of works from psychiatric hospitals, with his friend Karl Wilmanns. Although this originally started out as an attempt to assist diagnosis by examining the so-called 'psychotic art', it later became considered to be not just a tool, but true, valid art. Therefore, it goes to show that no matter what the work, or where it came from, you only need one person to stand up and say 'IT'S ART' for it to be so.
(I wonder if those poor sods ever knew that their deranged doodlings would one day hold such acclaim?!).