MDMA appears to amplify oxytocin signaling. Studies report increased oxytocin release after MDMA, and some evidence suggests the brain becomes more responsive to oxytocin during the experience.
Oxytocin is associated with trust, bonding, empathy, and feeling safe with others. As a result, many people on MDMA describe feeling warmer, more open, and more connected.
These effects are temporary and depend on dose, setting, and individual differences. Not everyone experiences them to the same degree.
For a concise overview written in plain language, see this resource.
Hope this clarifies it.