It is no different to people going off to fight in the Spanish Civil war in the 1930's.
The shared ideology and wanting to support people that you feel solidarity with are very similar.
This is not to condone such behaviour, but just to state that it is not something solely found in Muslims.
The situation in Syria and Iraq are very different. That most people seem to just think it's Moooslims fighting because of religion fills me with despair.
The uprising in Syria began mainly because increasing numbers of people were facing economic hardship because of government corruption and mismanagement. After 40 years of brutal dictatorship, they had had enough. This began in March 2011.
The protests were initially peaceful, but after being repeatedly attacked by govt forces and govt-backed militias, the protestors became armed and so the Free Syrian Army was born, with the aim of overthrowing the Assad regime and holding democratic elections.
There were also more Islamic styled groups, but they did consist of Syrians and worked with the FSA rebels, who still made up the majority of the anti-regime forces.
ISIS are from outside Syria. They spend most of their time fighting Syrian FSA and other groups. ISIS are actually helping Assad yallasouriya.wordpress.com/tag/isis-assad-links/ by fighting the rebels and dissuading Western regimes from supporting the FSA.
Now onto Iraq. One of the great calamities of the Western invasion was who they left behind to rule.
Former dictatorships with a diverse population are extremely prone to conflict and mismanagement. You need a leader who can unite society and bring a decent standard of living to all. Iraq instead got Noori Al Maliki, who has hugely favoured the Shi'a and brought in legislation to push this further.
This has lead to the Sunni population suffering from increased hardship and disenfranchisement. Therefore it is not too hard to understand why people might feel that an unjust regime forcibly imposed upon them, should not be removed by force. Enter ISIS.
I would state that it is not as simple as Sunni vs Shia. This to me, smacks of statecraft and that there are various countries trying to use events to boost their own power in the region.