Despite all of his training, Yoda still felt something for Dooku
Philosophy of old Jedi Order (before Order 66) was non-attachment, letting go of the things, those who turn to the Dark Side remain there etc ... Yoda himself, as Grand Master of the Order, held to those beliefs and publicly preached them many times to other Jedi (including Anakin, and much latter to his son Luke).
Yet, there are hints that Yoda never truly accepted Dooku's betrayal, especially not in such detached and definitive manner. In final episodes of Clone Wars (well, final before Disney start meddling :) ) Yoda twice meets Dooku during his Force visions. In episode Destiny Yoda has a vision of Jedi Temple in an alternate timeline without Clone War. Among the other Jedi there is Dooku, who acts friendly like nothing has happened between him and Yoda. Yoda almost accepts this vision, before finding strength to recognize it as an illusion. Then in Destiny, Yoda has a vision of him and Anakin fighting vs Dooku and Sidious. Anakin beats Dooku and proceeds to behead him (in the same manner he did later in real life). Yoda tries to stop him, in vain.
Both of these visions show that Yoda still had some attachments to Dooku, despite all of Jedi orthodoxy. Also, there was a premonition on his part that as long as Dooku lives, Sith still held something back. There was still certain sense of honor within Dooku, and only with his death things would finally deteriorate to a breaking point. We don't have explicit description of Yoda's reaction to Dooku's death, but after that he was very troubled by looming threat that was gathering over Jedi Order. Subconsciously, Yoda probably wanted to have Dooku back (first Force vision, playing on his wishes, shows that), only he could not publicly admit that. When Dooku died, all of Yoda's hopes died with him, including the hope that war could be resolved peacefully.