I always use "your" when labeling objects in a social context.
The way I see it, "your" reinforces the social interaction taking place here. As others have already mentioned, it is how another person would most likely address you when talking about your stuff. "My", on the other hand, imitates the user's point of view. It's as if the user went around their room with post-it notes labeling various items: my monitor, my chair, etc. Except, in this case, the user hasn't done this, the UI designer did it for them.
The whole point of XenForo is to encourage people to open up and converse with other users on the site, so it makes sense to avoid introversion -- and avoiding the use of "my" is just but one way of trying to accomplish that goal.