The Doctor flinched; he'd hurt a lot of people's feelings by abandoning them, but River was on top of the list of those he felt worse about leaving. Their relationship was on a level of intimacy that was only rivaled by the Master (strangely enough). She understood him, and what's more, she knew his name. The only reason she topped the list above the Master was due to their marriage vows.
"I didn't want to talk about it," the Doctor defended himself, more adamant than necessary. "Because that's what people would've wanted to do! Talk, talk, talk, talk, talk. Asking questions and telling me it's alright, and trying to get me to stay, or asking if they could come with me." He sealed his lips tight and then added, "It felt like a good idea at the time. I know it only made matters worse." A pause. "I'm sorry."