1. She really, really loves him. And vice versa. Neither is settling, and neither is rushing into a rash situation "to get it over with".
2. He has a job--more than one, actually--and he knows who he is.
3. Their families support them totally.
4. Her whole life will now be one of service. We won't see those parts as much as the appearances at friends' weddings and skiing holidays in Switzerland. However, most of the time, she will be going to duty after duty after duty, with cameras blinding her every time she steps out of the house. Every time.
5. Photographers and weirdos will dog her every step, and nasty-tongued comedians will make fun of her every chance they get.
6. I hope they will be at least as happy as I am. Ad multos annos.
Update (May 6): My own combox isn't allowing me to comment, so I will have to comment here. I note that some readers are unhappy that I am not embroidering a big 'F' for Fornication to sew onto the Duchess of Cambridge. There may be a cultural misunderstanding here. Although the Duchess of Cambridge is merely a "celebrity" to Americans, she is my future Queen. How Canadians like me--who derive not a little identity from Canadian (and therefore British) history--feel about our monarchs and their consorts may be a puzzle to Americans. However, let's just say that we are not particularly interested in chucking mud on them on their wedding day, if ever.
American readers might also not have realized that I will probably meet the Duchess of Cambridge one day, in the course of her duties and mine. She is not an imaginary character in a morality play. She is a living, breathing woman, serving the entire population of Great Britain (and to a certain extent the Commonwealth) in a very public way. Call me when Britney Spears embarks on a life of visiting hospitals and homes for the elderly. I live in Scotland, remember, and I move in many circles.
Meanwhile, I hope it is obvious that I do not think men and women should live together before they are married. However, in recent decades many Roman Catholic priests have told engaged couples that they can live together (usually for financial reasons) as long as they live together as "brother and sister." I don't know how likely it is that Catholics besotted with each other are strictly brotherly and sisterly, but neither do I personally know whether or not the Duke and Duchess of Cambridge lived together as brother and sister before they married. That they DID marry I think we can all agree is a very good thing. Neither had a good Catholic education; I think the traditional charitable phrase "invincible ignorance" might apply.
Incidentally, it was not the Archibishop of Canterbury who spoke so stupidly about cohabitation and milk and all that nonsense; it was some other Anglican bishop, and prior to the wedding. Normally I could not give two hoots what any Anglican bishop (except the very fine scripture scholar N.T. Wright) had to say, but give the Archb. of C. his due.
Finally, mote, beam: we all know the drill. For any more on the subject of purity, please see "Pure" in my book.
P.S. I deeply resent being accused of making excuses for, ahem, "celebrities."