More than likely a combination of your buddy learning to control aggro and your overhealing.
* Make sure he's not pulling w/ a Regen ticking away (Regen, Medica II)
* Throw a Stoneskin ... he should have some hate built up by the time it breaks for you to heal.
* Throw Eye for an Eye when it's up to reduce your need to heal
... or go ALL IN ... Throw an E4E (if up)/Stoneskin/Regen/ Cleric Stance/ HOLY HOLY HOLY the Bejesuz out of them Shroud/ turn off Cleric Stance.