Well I recently read a book regarding this by a Harvard scholar. It is true that Tamil Tiger suicide bombers are indeed secular, fighting for a homeland and all. However Islamic suicide bombers, are exactly that, religious. They are not however mad. Research shows that they are mostly intelligent, educated and sane people. This brings you to the question, what could possibly override their preservation mechanism to make them want to blow themselves up.
It is without a question of a doubt that there are factors which increase the the chance of somebody wanting to do this, i.e. religion, poverty etc. we only have to look at Palestine to see that. However religous dogma is exactly what provides many of these people with the sort of mental shield they need and in many cases the justification in order to press the button.
TomPaine