There are plenty of ways to introduce drama in a relationship and I agree, it is very rare to see romance stories that actually focus on them being a couple for longer than an epilogue and the worst, the first kiss in the last panel.
The best drama is having things your characters disagree on various topics/subjects; make one more dominant than the other in arguments/disagreements, where they win most and also choose for where they go for fun; have one be more open with their emotions and feelings with their S.O. and have the other be more closed in with theirs; have one get angry more openly while the other stays calm and deals with their anger differenlty; or better yet, have one that has a lot of self doubt and the other is very confident but hides their insecurities and see how they clash; and the best one, family interference that is not trying to set them up with a partner but more a negative influence on one while the other provides support where the family's minds are not changed but at least challenged.
There is a helluva lot of personal drama that comes up in relationships than a romantic rival. If they go through a rough patch have their closest friends urge them to break up with the other or the family do that but not some 3rd party.