Why did Feanor's sons take the Oath?

[with-rainfall.livejournal.com]

With all the discussion/speculation going on about the elves' motivations at Alqualonde, I'd like to throw this question out there: Why did Feanor's sons feel the need to simultaneously leap to his side and take the Oath? Was it truly a "madness" caused by grief over the death of their grandfather, family loyalty and Feanor's speeches? Was it an obligation to be by their father's side, or a sense of injustice, or simply the quasi-magical lure of the Silmarils? Was it more hesitant than the text paints it?

All of these? Or is there a simpler explanation, as in this Silmarillion/Good Omens fanfic?