Here's my explanation (copy and pasted from discord messages i sent):

i was thinking about gru and maxime being "exes" and i thought of a possible headcanon that could be tied to that

i think that neither gru or maxime were particularly popular when they first started at lychee pas bon - gru because he is generally offputting, and maxime because of the cockroach thing

gru would tend to keep to himself, while maxime would try to hang around more popular kids to try to fit in

at some point the kids would want to pull a prank on gru, and would use maxime for that because he was the least liked and they wanted to see how far he'd go to gain their friendship - that prank would be to ask gru to date him and pretend to be his boyfriend for a while. very mean-spirited, but something that should be expected from a bunch of future villains

maxime would ask, and gru - not realising it was a prank - would say yes. he'd be excited to have someone to hang out with

after a while of maxime pretending to like gru, he'd actually start catching feelings, as gru would be the only person to care about his interest in cockroaches. theyd be happy