Personally, I think the question of an individual fighter's motivation must be waived when discussing large groups. Not only are the reasons as diverse as the fighters and their backgrounds themselves, as it is extremely uncommon for anyone to record them (and even if one did the value of the data would be dubious - think job interviews, where the prospective employee tends to say what s/he think is expected) there's rarely any useful data available.

Of course, some of the techniques used by the sociel sciences rely heavily on specifically personal data gained through interviews and so on, but that probably isn't an issue here...