That's because he's had it drilled into his head, over and over, to protect me. He chooses me even when he shouldn't, even when it's self-destructive, even when it means giving up someone he loves. It's not because I matter more or because I make him happier. A lot of the time I make him miserable, because that's what family does.
Cas is family too. And there are other options besides choosing one of us to die. We're all getting out of this alive.
Do me a favor and pass that on to Cas for me, will you? If he's not already listening.