You see, according to US Weekly, two years ago (so just before or after their wedding?), Theroux found a secret stash of post-it notes that Pitt had written to Aniston back when they were together. You know the sort - cute little notes with sweet nothings like 'miss you already' and 'you look nice tonight' scrawled across them (side note for Brad, if you're reading: you need to lift your game).
US Weekly's source says that despite Jen's insisting that the notes "weren't a big deal", Theroux wasn't convinced. "Justin wasn't thrilled - he had moments of insecurity like that," they say.
And fair enough - if they weren't a big deal, why keep them?
Jen and Justin were together for seven years before announcing their split in February 2018. Jen and Brad dated for five years, between 2000 and 2005. In case you're wondering, this is why they shouldn't reunite.