A useful phrasing I picked up somewhere, years ago, is "I believe we are violently in agreement." That I agree with someone about a subject doesn't mean I'm always in a mood to talk about it. Maybe LW's friend was just drained, or maybe she knew that if the subject got to politics it would stay there all night, and there were other things she wanted to talk about.

Also, "we are all politically active" doesn't mean that they agree on every current, urgent issue. Sometimes "let's talk about something else" means the person doesn't want a nine-against-one argument. Sometimes she's carefully not asking her friends what they think about a specific issue, and really doesn't want to deal with that as dinner-party conversation.