Maybe they're just not thinking about the other person's feelings.
The fact they are actively trying to hide it could also be because they are trying not to hurt them.
It's hard to argue that they are getting thrills at the direct expense of an unknowing party, if they don't get caught. Of course, just putting the one you're supposed to be in love with at the risk of hurt is fairly horrible.
The cynic in me also just doesn't think it's sane to expect that an oath to love forever could be relied upon anyway, and the statistics do bear this out.
