You are curios as to the reasons?
Well, I'm all for the Aldmeri Dominion,
The theory most others will give is that because they live longer, they are much more 'wise'. I, for the most part, agree with this statement.
However, Altmer often live within the confines of the summerset Isles, and often live in luxury and under strict rule sets, and view their dated traditions as the only 'right' way to do things; combine this with their (possibly earned) high and tradition nature that stems from their culture, and you get a rather....arrogant race. Their queen is said to be a prime example of 'proper wisdom' as she's been out to explore the world and meet other races/people.
I, for one, am only in it for the pride of the khajiiti. (Potential future) Conspiracies aside, the Queen has brought support to the khajiiti people. Plus, garnering the favor of the elves (both bosmer and altmer) has never harmed anyone....especially if they win. That is not to say that i'm 100% sure that they will treat us that nicely, but....
I'd rather be at the foot of a filth-concerned primpy altmer, than at the pure scorn of a bigoted nord's muddy boots.
In other words: the Aldmeri Dominion seemed like the 'best' choice for me. Every other choice had less favorable prospects.
But as a player: Im only with the AD for the khajiits. Yeah, I gave you both a lore IC stand-point and a player standpoint. I couldn't tell which one the OP wanted.