What are the reasons for people to fall out of love?

Relationships take time, efforts, require nurturing, appreciation and love. When we are in a relationship we don't want to feel that we are taken for granted, cheated or misunderstood. When it comes to love it requires the presence of commitment, respect and trust. People easily fall in love but they face difficulty in staying committed and remaining in love. So, they fall out of love. Do you guys know what are the reasons for people to fall out of love?