There was a reason that the prolonged kiss between William and Catherine delighted the crowds outside Buckingham Palace so much; it was a genuine sign of love and affection from inside a family who have been trained not to show any in public, ever. Look at this photo from outside the Taj Mahal. They look like they've just met on a blind date, and they feel awkward.
Royals are discouraged from public displays of affection; especially when they're 'on duty', which usually involves representing the British Royal Family at an official function. They're expected to remain formal in such environments at all times, which is why you'll often see Royal couples stood an uncomfortable distance away from each other at official photo-calls, and it looks like everyone's fallen out. We're sure they still love each other behind closed doors.