Why do people declare love?

Why do people say they love us?
They promise things they are not sure they can actually do..they promise blue skies and butterflies and mornings enjoying coffees and warm hugs, just to get one day to see we were not realistic enough to get over the fairytales we want to live in.
Too bad because, meanwhile we get our way paved to disaster, to hate and blame that fairytale partner, we could have wine, heavy wild sex and spend nights in different cities of the world..