The Eldar have always intrigued me, perhaps due to their non-compliance with....anything. As the quote from Abriel said, they are a total random element, because they operate in prelude to events hundreds of years into the future, maybe even thousands of years. It's almost impossble to predict where, what and why the Eldar will appear. It is also impossible to predict what they will do. They are the epitome of fickle, making moves and maneuvers that suit only them. Their callous regard to other races like mankind is, in part, the only reason they survive. They have developed a macabre absolute sense of pragmatism, and their only concern is preserving themselves and their empire. 1) because they need to hold on to their past glory, and 2) because their demise only feeds Slaanesh. Of three things, the Eldar are constant. Their arrogance over races like Humanity and the Orks; their fear over groups like the Necrons and the Tyranids; and finally their tenacity in defending their own. Of all the races, I would least like to back an Eldar into a corner.
"He was the best of us. They struck without provocation, there was no reason. Animals! Brutal! They deserve no mercy! Strike them down, follow them back to their base and kill all of them, all of them! No mercy!" - Delenn