why people become so harsh & cruel to their loving ones?

why most of us become harsh, cruel, rough or unpleasant with whom we love most. we dont get time to enjoy with them. we - rather i - started taking her for granted. i do not agree with her in most of discussions or matters. i never tried to understand her, even i know i am wrong i stick to me obstinate nature. i miss her lot, but that is also true that i tease her lot, i hurt her lot. why its so?