probably because its easier to do romance in 1x1's. I've tried doing a side romance in a group with another player but it never worked out - there was either a focus solely on the action or solely on the side romance. It's hard to find a good balance without making the other players ill. But with 1x1s it doesn't matter what a third or fourth person might say - its just you and the partner who wants to do a romance.
Myself, I tend to go to 1x1s for the mushy stuff and great character building stories that usually last a good while. I go to groups for awesome action and questing adventures mainly because its more fun to have more players in those type of stories.