A good question. Depends on how much of Alistair's dialogue you opened up. He tells you a lot about himself and reading between the lines gives you some answers too.
First off, he grew up without a mother. His father didn't acknowledge his existence beyond foisting him off on Eamon to raise. He's told how worthless he is, that he is a threat to Cailan's throne. Eamon took care of him, barely, as Maric and Fiona wanted his parentage to remain a secret and so he couldn't divulge it to his wife Isolde. She felt resentment to the boy she thought was Eamon's bastard (and a potential threat to her own son's legacy). So she took every opportunity to belittle and torment the boy, finally convincing Eamon to send him to the Chantry. Let's not bother to ask Alistair what he wants. Just ship him off.
Along comes Duncan, the first person who cared what Alistair wanted. And what Alistair wanted was out. To be a Grey warden, something he was proud of. And here he finally found some folks who felt for the first time like "home" and "family" to him. If you took him into the Circle Tower, you find his nightmare isn't a nightmare at all, but a wish for acceptance and a home and family.
Everything that battle is tied to Loghain sticking the plan. Now I won't argue the validity of Loghain's departure from the field. Everyone except Loghain insists it was Loghain's departure that caused the loss, frankly I blame it on the three principles in charge in equal parts for different reasons I won't go into. Suffice to say I don't think Loghain should shoulder that entire fiasco's blame, but it succeeds in killing every grey warden in Ferelden except for Alistair and the HoF.
Everything done afterward however...
He races back to Denerim, seizes the throne, and then through threats and bloody violence tries to enlist the aid of the Bannorn against the darkspawn. Which fails miserably because he doesn't know diddly about how to talk to the nobles.
Afterward he systematically hires assassins and bounty hunters to harass the PC and his/her companions, sells Ferelden citizens to Tevinter blood mages, ignores the darkspawn sweeping over the nation to go after the rebellious noblity, captures and tortures innocents, poisons Eamon (who I don't particularly care for but Alistair is fond of), crawls into the 'political bed' with Howe of all snakes and generally puts the nation in a precarious state.
Add to this the crushing weight of 'survivor's guilt' Alistair is experiencing and that's a cocktail of misery that means Loghain is his 'breaking point'. He cannot conceive of inviting this man who has done so much damage and harmed so many into the organization Alistair loves (and frankly idolizes). Alistair's fury has blinded him to the fact that the organization is 'grey' wardens. Not white.
I would think someone who romances Alistair would have a hard time justifying sparing Loghain since over the course of the 'year' of the Blight, you would have grown close enough to know that there is no way he'd want someone who has spent the last year trying to kill you in the same camp with him. Trust just wouldn't be there.
You may have to save his recruitment for a subsequent warden.