With such a relationship going further, woman might have neurosis developing; she might have strong dislike or even disgust towards sex. Then she will start to find different reasons to avoid sex, as it was described in before mentioned letter – headache, tiredness, etc.
There might be many reasons why woman doesn’t like sex. Usually they are: poor technique of partner, short and not long enough foreplay, premature ejaculation, not big or wide enough penis, etc. There might be many reasons, but they should be looked at in context.
Answering to the question of the letter – why does she loose her interest in sex, I’ll answer – you are not giving your woman a sexual satisfaction. What to do in this situation? You have to seriously evaluate you’re nature gifts, technique and abilities. You have to get back that initial “sparkle” – ability to make your wife want you and bring her to orgasm.