The Maker is cruel, isn't it?
You and Alistair grew to be close. Have eachother's back for almost anything. Both of you were wanted, both of you were the only Grey Wardens in Ferelden for a long time. Yes, you both bickered but eventually everything did work out.
Alistair was happy to have someone he could rely on... after Duncan and the others were slaughtered and you were comatose in Flemeth's hut- he quickly realized how horrifying the idea of being the only Grey Warden in Ferelden would be.
And now. One of you had to die. Or, the ritual with Morrigan had to be done. Alistair was dreading the ritual a lot, yes. He thought at first you'd just tell him that you'd sacrifice yourself instead was just a comforting thought- you surely wouldn't do something as drastic as that right?
But no. Alistair's horror grew as he realized you were serious. Absolutely serious.
"You're really going to do this- you're not bluffing? Maker's breath I don't know if I can handle watching you get slaughtered by the archdemon or-! I'll do the damn ritual with Morrigan if it means saving you!"