In the episode "Loud as a Whisper" (season 2, episode 5) Worf was uneasy about meeting Riva because he negotiated several treaties between the Federation and the Klingon Empire, and Worf said that before Riva there was no word for "peacemaker." Why would he not be comfortable meeting with him if his past experiences with him were positive?
TROI: But you're feeling a certain confusion about this mission.
TROI: Yes! I've never known you have such strong emotions except when you're expecting to do battle.
WORF: I'm not expecting battle.
RIKER: Then what is bothering you?
PICARD: Huh. Riva negotiated several treaties between the Klingons and the Federation.
WORF: Before him, there was no Klingon word for 'peacemaker'.
RIKER: Then I can understand why you are feeling uneasy.
By the early stages of season 2 (S02E05 Loud as a Whisper), Worf had been shown to have wrapped himself in Klingon history and tradition the way a deeply religious person wholey embraces the dogma and rituals of their faith. He is about as open and accepting to the changes made to the Klingon cultural norm that (he perceived) Riva was responsible for as the Roman Catholic Church's faithful were to Martin Luther's reforms. A Psych 101 student would say that Worf embracing Klingon tradition was his way of coping with the loss of cultural identity and heritage he experienced growing up on Earth raised by human foster parents.
To further compound Worf's metathesiophobia, he felt powerless against these changes as he could not 'battle' them. Instead, he redirected his anger and apprehensions at who he considered the instigator; someone he was only peripherally familiar with.
During TNG's season 2 Worf was still pretty much a one dimensional character with limited growth and he was wrong here. In truth, Riva may have been one of the instruments of peace between the Klingons and the Federation, but he wasn't the catalyst of change. The single most influential diplomatic accord would likely have been the Treaty of Alliance (aka the Klingon Alliance), the principle arbitrator of which was Vulcan Ambassador Sarek, not Riva, although it's possible that Riva played a minor (unaccredited) role.
Of course, through TNG's 7 seasons and 3 of DS9's 7 seasons, the writers developed Worf into a much more complex and intelligent character. By the time of DS9's series finale, he had become Federation Ambassador to the Klingon homeworld although he felt himself ill-suited for the position and subsequently returned to Starfleet (source). Perhaps this move to the diplomat corp was an attempt to control the change he had previously felt powerless to prevent.