Do snakes bite humans without any reason?

Well if they have bitted without any reason, I would have died twice. I have myself went through very close encounters with Cobra, which is a venomous snake twice.

Once I was sitting outside the house, playing Mobile game. 'Game over' and I selected 'New Game' and realigned myself in the chair for more comfort. But then I noticed some suspicious object extreme left behind me, all of a sudden I jumped off from the chair. And looked back, it was a very lengthy Cobra, triggered by my jumping, it expanded it's hood and made that 'whoo' sound, and went away.
It was night, and I was talking via Mobile phone and noticed some very young Cobra, with not more than a 2 feet in length, went just by my foot.
In all the above experiences of mine, I learned something that is , we kill them whenever we see them, due to a possible threat of someone ever getting bitten , they tries to kill only when it feels as if we are threatening it's life .

Snakes are very morale in these things, much more than Humans.

But of course snakes can be stupid at times, misunderstanding our activities as a threat. If educated humans can kill people with gun in the name of 'Gods', how can you teach a snake what is a threat or what is not ?. Below are some circumstances where snake is very aggressive :

Never foot over a snake. It will attack you.
Like all moms, it's very protective towards it's offsprings or eggs, it's highly aggressive towards someone even walking by the eggs or offsprings.