Katie Holmes is divorcing Tom Cruise, ending another celebrity marriage that by Hollywood standards appeared to have staying power. What drove Katie to do it?

Holmes, 33, reportedly initiated divorce proceedings against Cruise, 49, who was previously married to Nicole Kidman and Mimi Rogers.

He has two children with Kidman, both adopted Isabella Jane, 10 and Connor Antony, 17. They separated in Feb. 2001 when Kidman was three months pregnant, but miscarried.

Cruise began dating actress Katie Holmes in 2004 and Cruise infamously declared his love for her by jumping up and down on Oprah Winfrey’s couch during her show. The were married in 2006 several months after Suri Cruise was born.

They made and interesting couple and Katie sacrificed several years of her career to raise Suri, who is now six. But last year she started working again in earnest. She starred in the controversial television movie about the personal life of John F. Kennedy and is working regularly again.

Tom has been just as busy on his “Mission Impossible” movies and filming “Rock of Ages,” the large ensemble movie based on the Broadway musical of the same name. It co-stars Adam Shankman, Alec Baldwin, Catherine Zeta-Jones and Diego Boneta.

The couple gave no reason for the divorce. “This is a personal and private matter for Katie and her family,” Holmes’s attorney, Jonathan Wolfe, told People. “Katie’s primary concern remains, as it always has been, her daughter’s best interest.”

“Kate has filed for divorce and Tom is deeply saddened and is concentrating on his three children. Please allow them their privacy,” a rep for Cruise told the magazine.

Professional and career differences could have driven a wedge between the two stars, which is often the case with Hollywood marriages. One or both may also have become involved in a “locationship” with another star while filming.

Cruise is also a fervent believer in scientology, and persuaded Holmes to adopt the pseudo-religion as well. Some of it’s quirkier beliefs may have been an issue.

There have also been occasional rumors, always hotly denied, that Cruise is bi-sexual. With the scandal that recently enveloped John Travolta over his homosexual escapades, could Katie have discovered some bone(r)s in Tom’s closet? Nah…