Why do men change after a few yrs of relation?

I hav seen dis in many cases dat men act vry charming at da beginning of a relationship but gradually as da relation grows old they b com cold [ they just cange] as if romance dosen't exhist n e more. Y does dis happen. where does all da love and excitement go. { oh btw i don't mean when i said excitement}. Excitement as in anticipation 2 meet ur lover or 2 date or talk with them