Xavier was just trying to talk him down. It's a thing that is known to work, appealing to the Phoenix host's emotions. It worked for Scott during the Dark Phoenix Saga, it worked for him during Phoenix:Endsong, and Wanda used it successfully with Hope. The reason it didn't work was probably in part the wedge driven between them by the whole thing with Vulcan, and in part that Xavier is just lousy at emotions.
