deleted scene (x)
Now this. THIS is a scene where I think Loki allows his true feelings for his brother to cloud his judgement. This is a scene where we see that Thor cares for Loki, and that Loki in turn, cares for Thor- it would have been much better for him not to go along with the plan- to plant the seed and ‘jump ship’ before it was too late. To let the usually egotistical, one-man-show of a brother of his, to go to Jotunheim all alone, and face the consequences in much the same way.
I don’t think it’s reluctance, or fear for his plans going awry that we see in the ‘what?’, in that expression… no. It’s genuine surprise. His brother, who has always been so strong, so capable of standing on his own - such a show off - didn’t even think it needed saying that his younger, obviously weaker, brother would be by his side in this battle. Loki is the one person Thor feels will be beside him without anything needing to be said; Thor needs Loki beside him- needs the reassurance of his brother’s presence (as we saw in the earlier deleted scene also - in which our ‘vain’ Thor asks Loki’s opinion of how he looks), and I think, perhaps, this is a moment in which, through all of his jealousy, all of his insecurities, Loki sees that, too.
Even just for this breif moment, before he returns to being a douche, Loki and Thor share a beautiful brotherly moment, and that wall of self-loathing Loki shrouds himself with disappears, for Thor and only Thor.