If spouses always had to file for divorce together, it would most likely drop the divorce rate to something close to zero. Your spouse's cooperation is not required when you file, even if you choose not to cite fault grounds in your divorce complaint or petition. A no-fault divorce is not the same as an uncontested divorce, in which spouses reach an agreement regarding all issues. In this case, they can file a petition together in many states.
If you file for no-fault divorce, this means you're not accusing your spouse of doing anything wrong to end your marriage. You simply tell the court that your marriage is irretrievably broken, or that you and your spouse have irreconcilable differences. In some states, you must live separately and apart for a period of time. Filing for no-fault divorce doesn't mean that your spouse agrees to give you the things you've asked for in your petition, such as custody or terms for property and debt division. It just means she won't contest your grounds.
Some jurisdictions allow you and your spouse to file for divorce together if your divorce is uncontested, provided you've reached an agreement on every possible issue. Some states, such as Ohio, require that you attach a copy of your agreement to your divorce petition. If you want to file for divorce on fault grounds, this would probably preclude filing a joint petition because your spouse would have to agree in the petition that she was guilty of marital misconduct. Joint petitions are typically based on no-fault grounds.