I sit next to my wife a lot, including about six hours in a car yesterday. I still like to sit next to her on planes because it makes the whole body contact and armrest sharing scene much more comfortable than it is with a stranger. It also ensures that there won't be a passenger of size on that side of me, though that's not the main reason. Finally, should we be on a flight with meal service, it usually gets me one and a half desserts!