Don't forget that's how famed couple Tom Cruise and Nicole Kidman first met... Days of Thunder. Another example - Ms. Alba is dating a personal assistant from her Fantastic Four movie. She was previously engaged to her Dark Angel co-star.
But Meg Ryan has the costar thing down pat. She's gone through actual/multiple marriages and divorces.
But anyway, if you think about it, there isn't much opportunity for these high profile people to meet someone and get to know each other on a personal level - other than when they're working with them. Same thing with people at work here hooking up because they work l00000ng hours - lots of married couples in our company. Anyway, it's easy for Hollywood to move on afterward because they don't see much of each other after their projects (onto the next one). Such is their life. Be glad it's not you... in some respects.