I am not having fun while having sex what could be the cause even when i try to have fun? may be that's why i am not having very hard erection. how to increase your sexual arousal?

Sex & the brain: Believe it or not, sexual response and fulfillment comes from the brain as its' source. The physiology of erection and performance is directly related to whether the brain "is into it" or not. Therefore, unless there is a physiologic reason such as poor blood flow to the sexual organ, then erections follow the lead of the brain response.