Have you ever gone to a museum and been so captivated by a piece of art that you want to photograph it? Chances are, if you tried, you would be approached by security who would politely tell you that flash photography is prohibited. It can seem like an arbitrary rule for some gallery-going patrons, but there are good reasons to disallow photography in museums.
The primary reason flashes are disallowed is to slow down the rate of deterioration of fragile artwork. Flash produces light and heat, which can cause a chemical reactions such as the breakdown of cellulose and pigment damage. A single flash is not a problem, but the repetition of many flashes over time could cause an art piece to deteriorate faster than normal. This is also why there are environmental controls in museums, to regulate temperature and moisture.
Another reason flash photography is disallowed is because it can be disruptive to other patrons. Museums are supposed to be a contemplative space, and a minimum of distractions can make a museum enjoyable for all who visit.
Some museums do not allow photography at all. This is usually due to copyright, when the museum does not own the pieces in question.
I would like to suggest an alternative to museum photos. Instead of taking a pic and moving on, put away the camera and have an experience with the art you see. Stare it down, let it move you. And stop by the gift shop to grab a postcard of it on the way out.