There are two answers to your question. 1.If it is put on the first part of the sentence, that means "It's because..".(very rough expression, used by men mainly.) 2."Datte" is casual expression of 'Desu'' and 'Da" and used as an Emphasis for the previous sentence. We sometime plus 'ba' "Datte" is used for men and women, "Datteba" is mostly used by women(a kind of pretty expression...not polite but not so impolite), and "dattebayo" used by men(very rough expression, sometimes impolite). "Who in the hell is in my room?""It's me, don't be so surprised!" ”私の部屋にいるのは一体誰？””私だってば、そんなに驚かないで！” （Watashi no heya ni iru no ha ittai dare?""Watashi datteba, sonna ni odoroka nai de!" "You said you would never come back right?""That was a lie, you know!” ”貴方はもう戻らないって言ったじゃない？””そんなの嘘だってばよ。” "Anata ha mou modoranai tte itta jya nai?" " Sonnano uso datteba yo,"