because he is stupid.
In my opinion in this situation there might be 3 cases :
1.it is the women fault because he lost his interest in her and then we can not say that she is that `` great women`` , right ?
2. the man is an idiot ,and he can not see how great she is ,and he does not deserve her anyway.
3.both of them have their own piece of guilt and in this situation it was just not meant do be betwen them .
End of story.