Just not interested anymore

The other day as I was putting my purse in my locker at work my friend told me that her and her boyfriend broke up. I use to cut her boyfriends hair and she's the one who brought him into see me before. I had asked her why they broke up and she said that he "wasn't interested" anymore. What the hell does that mean? Are we all in a relationship where all of a sudden your partner just stops being interested? What makes us all interesting to them in the first place anyways? And how can it be that all of a sudden they've lost interest?