Marriage and living together
When a person gets married or decides to live with someone they think their relationship will last forever.
However lots of parents find that after a while their love is fading away. People fall out of love as well as "falling in love".
If you asked all the children in your class about their parents you would probably find quite a few of them have parents who are separated or divorced.
This means a lot of children are growing up living with parents who live separately. Even if your parents do not live together you are still part of a family.