Why do most couples fall 'out of Love'?

At weddings, I often feel the passion and chemistry between the couples when they exchange vows, and it almost elicit tears that I struggle to hold back as a 'guy'.

Fast-forward three years, 2 kids are already involved, and she's looking 'blown-up', and you can't help but notice the quarrelsome tones they start using on themselves, sparks keep flying and four years later, 'He' and 'she' is Unhappily married.

Dear OAB readers, what do you think is responsible for couples falling out of Love?

Abeg make una tell me now o, I am still 'single' o ;-)