I suppose the whole point was to make it a bit mysterious. To make that line ''foolishness'' etc more up to interpretation than it would be if they just said ''oh yeah, Vergil wants power... just because''.
You could also say it was Vergil's fate to be the opposite of Dante, just like Sparda had two sides to him: the human-like side, and the demonic side. Vergil is Sparda's demonic side, while Dante represents his emotional side. The emotional, human-like side always prevails, which is DMC3's theme as well as DMC4's. It's just that DMC4 focused on the 'love' aspect more. That would indeed make DMC pretty much lack character motivation.
I'm guessing the Japanese don't like finality as much as we do. We always want answers for everything, whereas I often get the sense that a believable story is enough for them.