Ituvia....it sounds like he was motivated by whatever felt good in that moment. It probably felt good when he implied marriage to her...and, it kept her on the hook....
It probably felt good when he promised "forever" to you, in that moment...because it kept you on the hook....
Immediate gratification....his gratification....
Just like a drink provides immediate gratification....
You do remember that alcohol messes up the frontal lobes...where logic. planning, and many other executive functions reside....it is sort of like a chemical lobotomy....