Question: how can a man fell out of love with his girlfriend of 8 years? 

Answer: Although there could be several answers to this question, one might want to ask why after 8 years are you not married.  The same thing that attracted him to her has to somehow still be in the picture.  Relationships can go cold, but each person has to remember to respect, trust, and keep the relationship spicey.
This question from the Yahoo Q&A forum is categorized in the topic: Science & Mathematics