It doesn't work like that at all. MDMA gives you a sense of euphoria while slightly altering your perception (it's considered a slight psychedelic in that sense) - it also makes you very empathic to the people you are with. MDMA isn't like a love potion, usually, once you come down you go back to being "you". It doesn't make you addicted to the people you experienced it with at all. What it
does do is open you all up, so at
controlled doses that can be highly beneficial in a therapeutic setting because both people will feel love for each other and empathy. Which, at a controlled dose, they should also be able to coherently iron out their issues.
Once they come down, they go back to being who/what they were, but with a whole lot more "sharing" now - they both now know how each other feel because it was expressed out of love and empathy rather than witheld or stated in a defensive/offensive manner.
Psychedelics really shouldn't be toyed with unless you are psychologically stable - if you can imagine, being "normal", then psychedelics take you to the reciprocal of "normal". I've played with Magic Mushrooms and really hated the experience, I also did two nights of Ayahuasca - extremely powerful experience, beyond LSD for sure, but a very rough ride (Ayahuasca is a shamanic "purging" psychedelic, it causes you to vomit uncontrollably while having the hallucinogenic and psychedelic experience of LSD x 5 for about 8 to 10 hours straight).
I will say this: I believe all of the benefits people would seek in the use of mind-altering substances can, without question, be found through meditation - deliberate thinking - introspection - and just "chilling the fuck out".