i have had times (rare) where i would almost faint or have the sensation related to dissociating...but fainting generally is due to lack of oxygen to the brain.

i have fainted in the past due to a combination of not eating enough, being dehydrated, and a medication which was a sedative type...i also had low blood pressure which would cause me to feel light headed and almost feel like i would faint at times.

sometimes even anxiety/panic can be a cause for fainting due to shallow breathing. i had my vision go black once from a panic attack..it was really scary.

so, those are all some potential things that generally could cause fainting.