Don’t know it this has been put up yet, but Eridan actually wanted to help Kanaya with the matriorb (before his meltdown, of course). He viewed the gesture of restoring their race as “hopeful,” and thus wanted to involve himself. (This becomes quite clear when you play as Eridan and speak to Kanaya directly.)
(This happens in “[S] Kanaya: Return to the core.”)
http://www.mspaintadventures.com/?s=6&p=005221