Close proximity
They're close together on the set 16 hours a day, 6 days a week, for about 3 months. Not much of a life for the leading actors when they are in principal photography. The actors sometimes go out drinking with some of the crew. But the actresses don't really hang out boozing with the crew, so many of them are only left to be attracted to the leading man.
She has 2 hours of hair and makeup in the morning so she looks like a knockout. She's on the rebound from puff daddy. More than enough of a recipe. Flings are very normal. Engagement, just the next step. Marriage, hard to last.