Sheetraj,
to answer your question, which the others seem to have forgotten to do, ther is actually no reason why a commercial pilot should not join an air force (or army or navy). The main reason why very few do is that they are too old. To take the Royal Air Force as an example (I'm sure the same is true in most countries), the maximum age to join as a pilot is 23 1/2 years old. That's pretty young, and ensures that the military get a fit, enthusiastic individual who is likely to have few commitments and fewer responsibilities - in other words, he'll take the risks to get the fun! Most commercial pilots a) are over that age by the time they're qualified and b) are far too sensible to want to risk their lives in the military!
I'm not trying to justify the case, just state the facts as I see them. Hope this helps!