Stops you castling. Maybe they're playing for a draw, or fancy their endgame skills.
I don't get Queen for Queen

If the the capturing queen has to be retaken by the king. I will usually do it just to prevent castling and keep his/her king in the center. The trade would not be completely even. Though material is even, the side that initiated the the trade would have a positional advantage. If the queen can be recaptured with a Knight on c3 or c6 or something else I will think about the trade a little longer but usually I will still do it b/c an opponent's knight on the back rank (d1 or d8) is usually better than having it exert pressure on the center from c3/c6. Rarely are trades of equal material completely even often they result in a positional or tactical advantage for one side.
But then again I'm just a patzer, and don't realy know what I'm doing in a chess game.

1. Stops castling.
2. Normally good for tempo - moving the queen to get yours to safety often will cost you, especially if your opponent is chasing it. Instead they will likely have to move the king to safety.
3. If you're up material simplification will give you better winning chances, and decreased losing chances.
In a few games lately if both players Queens are exposed when they are still next to the King, the other player will go Queen for queen.
Now I understand if it's for position, etc.. but it's always just for the sake of taking the queen even though they always lose theirs.
Am I missing something?
I just don't get it.