Maybe I'll leave the Civil War quest line alone on my second playtrough. What would be a reason to join them up? I finished Storm Cloaks on my first run.

Bosmer are from Valenwood. Valenwood used to be part of the Empire. The Thalmor took control of Valenwood and reformed the Aldmeri Dominion. Some Bosmer are probably not too happy about that. Some Bosmer liked the Imperial rule and want to return to those times.

Pretty much same as why any other race would want to join the Empire. The Empire was meant to bring all the races together.

for whatever reason this person would have, you're the one making his personality and past you know

the altmer occasionally purge valenwood of undesirables people who dont support the thalmor talos worshippers and other people they deem unfit to be a part of the thalmor.

The Empire had their laws but pretty much let people do what they wanted while the Thalmor are far more restricting and kind of racist overall with their High Elf elitism.

Same reason anyone else has to join the Imperials ... the stormcloaks.

Stormcloak offended you with racial taunts; called you short.

Cue a Marty McFly challenge accepted.

If you wish to have any level of realism in your play, joining the Stormcloacks is not an option. You would die from hypothermia in the attempt.