Wait guys are you really seeing “i love all of you” as castiel trying to cover up the first i love you? That wasn’t a cover up, that was a confession. Let me explain.
Cas says “i love you” and looks down(that is the only time he looks down, except when he’s thinking of what to say), camera cuts to dean and dean is.. a bit annoyed by it?? Because he knows where this is going. Cas loves them and he’s dying and dean is tired of all this saying goodbye crap. He’ glad he hears these words, but why are they always said in situations like these?
bUT THEN MY FRIENDS, cas says “i love all of you.” Camera once again cuts to dean, but this time dean doesn’t move a muscle. There is no need to say it twice, unless you’re one of these two idiots can’t confess their love without getting family in it. But they do say it one way or the other. He realises this is how he said that to cas and now he knows the first “i love you” was only for him. (May i add, i don’t think anybody ever said to cas they loved him except dean and dean said it this way. I might be wrong but..) It honestly looks like he’s caught off guard, he thought the L train has passed(tf am i even saying, but you get the point). And guess what, sam notices too. He’s heartbroken because his best friend is dying,obviously, but he knows cas loves them, he knows cas is family. He also knows what dean means to cas, and cas to dean. He’s spent too much time with them not to know.
Idk, to me it seemed deeper and more honest than cas just hiding love for dean in his last moments. They all already know so what’s the point. I also think this was a response (a very ironic one), to “we need you. I need you.”