In many states, couples who are getting divorced must allege - and if contested, prove - that a divorce is necessary based on a ground laid out by statute. Examples include such things as adultery, bigamy, abuse, incarceration, etc. Other jurisdictions recognize a "no-fault" divorce. In these types of dissolution proceedings, the couples basically just agree that they want to get divorced and file the necessary petitions.
Washington residents do not have to allege any specific grounds for dissolution other than that the marriage is "irretrievably broken." This breakdown must be alleged in the petition for dissolution that is filed with the court. If the non-filing spouse agrees, or joins in the petition, with the other spouse, the court will make a finding that the marriage is irretrievably broken and enter a decree of dissolution.
Washington does, however, require a 90-day "cooling off" period after filing for a divorce. Once the petition has been filed with the court and appropriately served on the other party, the couple must wait 90 days before the court will enter a decree of dissolution. Avoiding this waiting period is one reason a couple may opt for legal separation in lieu of a dissolution.
If the parties do not agree that the marriage is irretrievable broken, the court will consider all of the facts that are relevant to the case, including whether or not there exists a potential for reconciliation as well as the circumstances that resulted in the filing of the initial petition. The court may then enter a decree of dissolution or transfer the case to family court, refer the couple to a counselor and review the counselor's report after a 60-day continuance.
Source: Washington State Legislature, "RCW 26.09.030 - Petition for dissolution of marriage or domestic partnership-Court proceedings, findings-Transfer to family court-Legal separation in lieu of dissolution.," accessed April 24, 2018