Well I mean Nurgle loves Mortarion for sure, Mortarion was less then pleased to have to swear to Nurgle. I dont remember the specifics, but from what i remember reading it essentially goes that Mortarion was getting pretty paranoid by the end of the heresy and so he had the navigators on his ships killed cause fuck witches (its kinda his thing, thouhg it is understandable since hes one of the only primarchs with an actual frame of reference for how dangerous and fucked up psykers can be) buuut that presented something of a problem on getting to Terra to siege the shit out of it. Someone, probably fucking Typhon, said they had another way to get there which i forget what it was, and so Mortarion had them all set up to head to Terra and siege the shit out of it. However fucking Typhon, Mortarions first captain who was a secret witch or something and had given himself to or got corrupted by nurgle though im pretty sure its the former, lowered the gellar fields during transit and Mortarions ships were easy pickings for Nurgle to fuck over at that point. The death legions famed endurability kinda backfires on them at this point as it keeps them barely alive under the worst agony imaginable. Nurgle said he'd let them all live if Mortarion swore himself and his servants to Nurgle, and Mortarion agreed in order to spare his sons any more suffering, cause although hes a grumpy bastard he apparently has a soft spot in his grumpy heart especially for his legion which seems to be a common thread among primarchs. They get Nurglefied no take backs, fuck up loyalist scum and get fucked up by loyalist scum in return. Horus dies, Traitors beeline for the eye of terror, and Mortarion essentially ends up becoming everything he loathed about his foster father, without ever getting a chance at fucking up his real father. Cue eternal sulking and despair, and Nurgles got his perfect subject/'child' in the bag. At least that's my understanding of it, but id recommend checking out other sources, specially 1d4chan which has a lot of funny articles that can help point ya in the right direction vaguely.
On a small side note, I wanna mention that I actually find Nurgle the creepiest and most loathsome of the chaos gods. Mostly because he just reminds me too much of a creepy stalker that keeps people locked up in his basements cause he loves them and wants to protect them, and doesn't give a shit if they like it or not.