-아/어서 is most common, and -라서 is used with a noun. A : Why did you pick this up? B : 맘에 들어서. Because I liked it. A : Why shouldn't you be in love with the guy? B : 내 친구 남자친구라서. Because he's my friend's boyfriend. -니까 is more emphatic and has a certain tone like "That's why..." or "One's supposed to...". (A's already tidied up the house but B's still preparing dinner...for 3 hours) A : HURRY UP PLEASE, WILL YOU! B : Why are you so upset? A : 배고프니까! Because I'm hungry! (that's why I'm angry!) A : Why are you...so sweet? B : 사랑하니까. Because I love you. (that's why I'm so sweet / one's supposed to be if one's in love with another) -기 때문에 is used in formal writing, we hardly use it in daily conversation. Only 때문에 is used with a noun in everyday Korean. 너 때문에 내가 못 살겠다. Because of you, I can't live. (means "You're such a pain to be with") 비 때문에 못 가요. Because of the rain, I can't go.