Like a magnet, polar opposites attract.

Differences and opposites pull, attract and draw together while samenesses and similarities push away or repel from each other

So opposite people fit together and identical people don't mesh together, according to the expression.

When someone goes to the gym and spends 90% of the time on their phones scrolling through social media

The epitome of people that should not be together but in actuality are. Example: Taryn and Cassandra.