^Or maybe Fergus had better taste. ?
It was stated early on that Teagan never liked politics the way Eamon did. It's possible that he was fine being a Bann but never wanted to be an Arl. If Eamon is in Denerim advising the monarch, and left Redcliffe's Arl-dom to his brother, Teagan might not be altogether happy with that elevation/arrangement. Add to that all the extra damage from the Blight that might still linger, the damages from the mage-templar conflict, the tension of the mage occupation, the rude eviction by foreign 'Vints, and the addition of the Ambassador duties to the Conclave, it's easy to see where the previously easy-going guy might be barely civil. It's sad to see, but not incomprehensible.