Once a couple fall in love, they rarely stay in love for long.
Why?
Could be because of a number of things:
People are just cheap.
Love comes easy (well, shouldn't it?)
Infactuation is a temporary thing that eventually melts away (like ice cream).
Familiarity kills curiosity.
Passion and exciting things rarely last.
Being in love (or doing anything) all the time is boring.
There are other (and probably more) exciting things around.
Etc.
If being in love is better than falling in love, the logical conclusion would then be never to get married, or if you two have to get married, not to live under the same roof (but instead next door, block, or street).
In other words, keep a distance to keep love fresh.
Love at first sight usully doesn't last. Love at a distance does.