Also, keep in mind neither the Warden nor Alistair knew that a Warden has to sacrifice themselves at the end. They didn't learn that until the Landsmeet was over and they were back at Redcliffe. I honestly like to think that had Alistair known that he would have been more willing to let Loghain join, if for no other reason to be the one to die. Especially if he was good friends or in a relationship with the Warden.
I agree to an extent, but IMO it comes down to immaturity and stubbornness on the part of Alistair. Even though he couldn't elaborate in front of everyone at the Landsmeet, Riordan said that there were "compelling reasons" why they needed as many Wardens on hand as possible. Rather than accept the word of a senior Warden, Alistair ignores this and throws a tantrum instead. I can understand why he did it, but it really does highlight some of the flaws in Alistair's character. He should have taken Riordan at his word or at least given him the opportunity to explain himself later and then decide, but instead he decides that his personal feelings are more important than ending the Blight... that's inexcusable considering the number of lives at risk if the Archdemon was allowed to roam free.
Not that I'm complaining, of course... I think that this critical flaw in Alistair is what makes him one of the best characters in the series. Not all heroes are capable of overcoming their faults, after all...
I'll also add that it's clear from the start that Alistair doesn't actually agree with the idea that Grey Wardens should do whatever they must to achieve their goals... he disagrees with pretty much every morally ambiguous choice in the game and only approves of the most altruistic of actions. Although he rarely openly challenges the Warden's unscrupulous decisions, it's clear that he sees the Order very differently from how they actually are. It makes sense though, since he had only been a Warden for a short time before Ostagaar, and so he never got a chance to learn the truth about his comrades-in-arms... how they became Wardens, what their lives were like before the Joining, what kind of things the leaders of the group were involved in between Blights, etc...