A source told the publication, 'He stumbled upon old Post-it notes Brad had written.
'Sweet little Post-its like, ‘You looked nice tonight’ or ‘Miss you already.’'
The source adds that those notes really affected Jen and Justin's relationship.
'Jen assured him they weren’t a big deal, but Justin wasn’t thrilled…Justin had moments of insecurity like that.'
As previously reported, Aniston and Theroux announced their separation via a joint statement earlier this week: 'In an effort to reduce any further speculation, we have decided to announce our separation. This decision was mutual and lovingly made at the end of last year. We are two best friends who have decided to part ways as a couple, but look forward to continuing our cherished friendship.'