What causes people to constantly test their romantic partners?
The hard work of creating and maintaining great relationships is an ongoing process. Like any important investment, it will only show a profit if it is continually innovating and transforming.
Successful relationships depend on the willingness of both partners to work on deepening their mutual understanding of what each can give and needs in return. In order for that process to work, they must continually guide one another towards successful awareness.
Sadly, this process fails when one or the other partner not only doesn’t give helpful feedback but actually tests the other in ways they cannot succeed. Partners who are repeatedly thrown off-guard in these ways gradually lose their grip on reality as they continually fail at gaining approval from the other.
It would seem so likely that people who profess to love their partners would do this to the other partner, whether purposefully or not. It is also hard to believe that the partners who are continually tested would be able to tolerate that gaslighting for any period of time.