Why the hell shouldn't she grin all the time? Rarely has a stalker on the prowl for a Royal husband been as successful as Kate, with assistance from Ma Midds.
She probably still can't believe that all those strategy meetings about where William was going to school and spending his gap year-so that she could conveniently run into him-bore such spectacular fruit.
In all seriousness, she has probably been advised by her
advisors at St James Palace and Clarence House to smile, smile, smile at all times, lest the disgruntled common folk get the idea that you are unhappy or ungrateful for your good fortune.