HAFTA

"Deriving from 'have to', or 'have+{infinitive}' broken down into 'have+to+{verb}' but minus the verb, 'hafta' works as a verb-modifying adverb taking the place of the modal-auxilary 'must' which means literally 'to be required to __'. In colloquial speech (as opposed to 'formal speech' under which you would not use this word{hafta}}, it means simply to have a strong desire towards, or want for, doing {something}, with the 'something' usually seen through the eyes of the speaker as a *necessity* (even if in actuality it is no more than icing on the cake)."

That's why!