We're including Dobrev and Somerhalder together, because theirs was a mutual decision. The pair are a former real-life couple - a case of life imitating art as their 'Vampire Diaries' characters were also in a relationship. Dobrev and Somerhalder couldn't make their love last, although they remain good friends. It did make kissing feel strange after their break-up, though, and so they pushed back against any such scenes in the show going forward from that point.
Their decision did take some of the heat out of the Damon-Elena relationship on the show, although their decision to opt-out wasn't absolute. In the very final episode of the show - when the moment called for it the most - they agreed to do one final kiss. Fans of the show - who knew of the drama behind the scenes - appreciated them for it.