Why is sex feel so good.?

To make babies!: "why" questions are hard to answer in the natural sciences, as they imply there's a purpose or intent. Biology usually relies on evolution principles to answer "why" questions. In this case, the good feelings from sex motivate the behavior, resulting in propagation of the species. If you seek a neurological explanation (how not why), perhaps a neurologist can answer.

Very difficult: very difficult question since we do not know you - many factors can contribute