Sometimes one partner is violent and abusive and the other must flee to safety (with the children if any) and use a mediator to obtain the divorce.
In many cases, one partner cheated and committed adultery (had sexual relations with another person outside the marriage), sometimes both partners will have had affairs. Sometimes they cannot forgive one another.
Sometimes people get married too quickly without realizing they’re not really compatible.
Sometimes one partner finds a dream career far away from where they have their marital home, and the other knows he/she absolutely cannot move.
Sometimes it’s after the loss of a child: by abduction or death by accident or disease, the process of intense grieving drives a couple apart rather than drawing them together. Some marriages don’t survive tragedy.
Sometimes if one partner is very ill or is in an accident and becomes handicapped, the other feels overwhelmed and flees the marriage, compounding the trauma experienced by the other.
And sometimes one partner joins a fanatical political or religious group and the other cannot draw him/her away from it and must let go.
Sometimes the love is neglected and quietly dies.
In any case, divorce is a life-changing experience and both parties need time to heal, to be free to love again with someone new.