I can only give an estimate about the UK.
It is important to note initially the main emphasis was going to fight in Syria, not Iraq; nor until relatively recently was ISIS the main destination.
Each case is likely to be different, whether it is an individual decision or a small group.
During the Syrian Civil War helping those under attack by the regime was the reason, allied with the apparently steady advance of jihadist groups who stressed jihad and then the pursuit of an AK47 adventure. Once it became apparent this was a horrible civil war, where fighting each other was more likely than fighting the regime, becoming a jihadist came to the fore.
My estimate is that with the public arrival of ISIS there has been a change as the "message" about the battlefield(s) in The Levant being the final battleground emerged. I have doubts that this message even featured before, very few Muslims appear to have acknowledged this factor.
One snag is that to date very few, if any, veterans from Syria have actually talked publicly - when hopefully their disillusionment would be to the fore. It is interesting that some groups and older veterans have advised against going.
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