Why do we hurt each other so much??

i hear people say all the time "i didnt mean to hurt you" but deep down didnt you know that you would eventually hurt the person? in cheating when that is said how could u not mean to hurt the person? and to add to that you say it didnt mean anything? in the case of lying, u knew the information was vital yet harsh, but you still didnt tell the person, how would you could you not know it wouldnt hurt the other person? i cant seem to grasp the concept "i didnt meant to hurt you", like if you accidently hit someone, you dont even say i didnt mean to hurt you because it was so involuntary...but you get what i'm saying?? i dont know if it's jsut me but it seems that the more we say we love each other the more we seem to have the liberty to hurt them, because if we ACTUALLY did love one another, your purpose would be to protect, love and cherish each other...i just dont get it u know.