I don't really have a favourite colour. My dd quite often asks what's your favourite colour today, mummy, and we both our colour of the day 
To the letter - I think this is more common than people admit. I think some people drift into marriage because it's the right 'time' in their lives, and the person they're with is 'ok' but, they're not in love with them. I agree the letter writer sounds like it could be an arranged marriage though.
When I worked, we were sent on the soul-searching type training course, called "Making the Difference'. It was an incredible course, all about what makes you happy/unhappy, and what you can change in your life (Lord knows why the company paid for this!).
One bloke at the end stood up and said 'I have never loved my wife, and I have realised I need to end my marriage'. You'd never have known he wasn't happily married.