#1 Issues between the two ruins their on-screen momentum
Anyone who's ever been in a relationship knows that sometimes, during a bad day, that's all that plays in your mind. It becomes impossible to focus on anything at all during such occasions.
Maybe Vince McMahon stipulates that Seth Rollins and Becky Lynch will be on two different brands for the next six months. And it is this distance, this longing for one another, that begins to eat into their performances, because of which each of them loses their respective matches.
If WWE has the patience and the creativity to tell this story, it would really be an interesting long-term arc between the two. It would also be something far removed from what WWE usually does, which could make for some interesting television on both brands.
WWE could also just showcase a simple backstage argument as the reason for the rift if they wish to do so.