This application form was clearly produced by a pre-school.

Or Graduate from the School of Stating the Bleeding Obvious. It reminds me of the US landing forms that ask if you are currently or have ever been actively engaged in terrorism or genocide.
To Marry my Fiance and settle in the UK is a safe bet.
To live happily ever after is optional..
Interesting that it wants the 'main' reason.
A very strange question. Hope they're not all as pointless...
Good Luck anyway.