There are various reasons why someone might need to file a single petition for divorce:
1. Irreconcilable Differences: The marriage has broken down irreparably, making it impossible for the spouses to continue living together as a married couple, but one spouse is unwilling to agree to a divorce.
2. Abandonment: One spouse may have abandoned the other for a significant period, making it necessary for the abandoned spouse to seek a divorce.
3. Adultery: If one spouse has committed adultery and the other wishes to end the marriage as a result, they may file a single petition for divorce.
4. Unreasonable Behaviour: If one spouse's behaviour is unreasonable or intolerable to the point that it has caused irreparable damage to the marriage, the other spouse may seek a divorce through a single petition.
Ultimately, a single petition allows one spouse to seek a divorce unilaterally when the other spouse is unwilling or unable to agree to the dissolution of the marriage.