It's part arrogance, part experience.
(The following is Disney Canon)
Both Obi-Wan and Anakin have grown between the end of Episode II, where they got beat pretty badly by Dooku, and Episode III, where Obi-Wan still gets beat pretty badly, but fortunately Anakin is way stronger.
During the Clone Wars, the two of them battled against both Count Dooku and Darth Maul on many occasions. On each of these occasions, while neither side was able to ultimately kill the other, Obi-Wan and Anakin were definitely on the winning side.
With the clone wars reaching their end in Episode III, it's no surprise that Obi-Wan was feeling confident. His enemies were on the ropes and the republic were mopping up what was left of them. So when Dooku walks in, Obi-Wan doesn't see him as a powerful Sith Lord to be wary of, but rather somebody who hasn't been able to kill him, even with help from Darth Maul, over the years. There's also the fact that Super-Anakin is with him on this occasion as well, so where before Obi-Wan might still emerge victorious in a 1v2 against Sith Lords, he's now in a 2v1 against just Dooku. The odds are certainly in Kenobi's favour, or so he believes.
Basically, his reaction amounts to "Oh, for the love of.... Dooku again? Be right back, Chancellor, we'll just deal with this, then untie you, won't take a sec, kicked him and his buddies to the curb hundreds of times, don't you worry."
Of course, this arrogance is probably what led to him getting smacked across a room and knocked unconscious.
TL;DR - He's fought and won against Dooku multiple times during the Clone Wars. To Kenobi, Dooku is more of a thorn in his side than a worthy adversary at this point.
BONUS OUT-OF-UNIVERSE ANSWER: It's yet another Ewan McGregor ad-lib.