Two general categories:
1) I notice a LOT of middle aged, fairly unattractive white men with YOUNG asian women with children in tow. These are the men who somehow missed the boat and this is their only way to start a family. Think mail order bride.

2) Small petite delicate asian women make white men feel more masculine, esp. the shorter smaller men.

And then there are always the ones where they were just genuinely attracted to each other.