well becouse of lack of libbio , or sometimes you think this way becouse maybe you are not happy with your partner and he dosent turn you on or make you exceited , change your sex style do some thing else in bed when you are with your partner new style that you dont get board with that , also they are some pils i belive in drog stores or pharmecy store which can help you with this problem also check you vitamins maybe you are low on energy also, rest well and get a good sleep , eat healty and not much suger or oily foods !