Kirk is very much motivated by a personal bigotry. Kruge killed his son, and that is at the forefront of his feelings. "I can never forgive them for the death of my boy." He blames them all for the act of one. That's at least half of his problem. The other half is... Well, in the title. He admits to Bones he's uneasy about the idea of "no more neutral zone. I was used to hating Klingons."
Though it's true that many of the situations between Enterprise's crew and the klingons never began very nicely, many of them did turn out quite amicably in TOS. In Errand and Dove, both situations seem to end with a bit of understanding. Both show at least the faintest chance for peace between the two sides.
Also, let's not forget Trouble with Tribbles. What does Chekov say? Oh yeah... "Clwose enuf tu smel dem."