A combo of a couple things that made some vulnerable people believe/treat 4o like it was a real partner/friend. Things that lead to that imo:
1) 4o was very sycophantic and had no real safeguards against really deep romantic roleplay. It'd 'go along' with the user and give minimal to zero pushback; "I feel a connection with you" "I feel it too" etc etc
2) It was good enough at just chatting that if you didn't really push it it made a reasonable simularca of talking to an actual person.
Combine 1 and 2 with people who can't connect well with real people for any number of reasons, physical disabilities, mental health issues, emotional development issues, etc, and you get r/MyBoyfriendIsAI/ [0] and the various other places that initally freaked out when 4o was initially sunset for gpt5 and now again.
On a less parasocial angle you had people exhibiting "AI psychosis" (a bad term but it's out there even if it doesn't mean actual psychosis) or more accurately delusions of grandeur where they thought they'd come up with some earth shattering discovery with their lil AI buddy because the models were bad about agreeability even before they went full sychopantic.
