Stone is always thrown at the fruit that is ripe and tasty to eat. It is same in the case of relegion. Islam as you might be knowing is relatively young relegion in its present form after Prophet Mohammed. It just started in a small place in Saudi Arabia and has spread to the whole world within 1500 years. But other relegions though have been for many more centuries have not attained the same growth as Islam has attained. So people of other relegious faith attack Islam. When an individuals relegion is attacked he does not keep quite and starts to fight against the oppression which is the real cause of Jehad which you can see in various forms as seen in Afghanistan,Kashmir, Iraq, Palestine etc
