In neon genesis evangelion why does shinji run away?

I haven't personally finished Neon Genesis Evangelion, but I believe Shinji ran away after all the trauma he has experienced over the last couple of days. The way he was treated and berated for not being able to pilot or pilot correctly, having to do a multitude of chores, his overall feelings of inadequacy and puberty...Poor kid needed a break.

Generally spontaneous human combustion is something that still bewilders some, human combustion happens very rarely and doesn't affect the surrounding areas, so unless you want to set your self on fire, human combustion will remain spontaneous and very rare.