|Why does the snake does not die from its own venom?|
The reason is that they have separate venom producing glands. The venom is generally made up of amino acids. A small coat of sugar crystals cover the outer surface of the gland which does not dissolve in the amino acids. So even when the venom is present in the glands they prove to be harmless to them. The venom is only harmful if it combines with our blood stream. Generally we die of venom because the amino acids present in the venom combines with the blood stream to clot our blood. So our blood flow stops and as a result death occurs.
You may have another question arising in the mind. You would have seen a snake eating another snake or heard of people eating snakes. Then why don't they die? The reason is more or less the same. If we eat the snake, and till it reaches stomach, there is no contact with the blood. And in the stomach strong acids dissolve the venom i.e. amino acids to convert into simple salts. So after that the venom is just like normal salts.
A suprising fact:
At last a suprising thing is that you must have seen mongoose does not die easily even if it is bitten. The reason behind this is that even the glands of the mongoose are sugar coated. So till the frag of the snake pierces through, to enter the blood stream, the mongoose doesn't die.