People who are stuck in the oedipal stage can’t avoid forming conflict-ridden triangles. The point of this activity is to try to insert oneself into a dyad, proving that you matter more than the relationship within it.
Example: you love your job / hobby / friend Judy more than me! Do I even matter to you?
Another example: mommy can’t possibly have sex with daddy, she loves me and not him. Yesterday and today, my news feed is exploding with missives from sad children who are still fixated on mommy.
The main two tasks parents face are teaching the child to deal with frustration and helping the child to see the role of “an excluded third participant” as acceptable and not painful. Many people fail at this task judging by the crazy proliferation of triangles everywhere.