Like all U.S. states, Washington is a no-fault state. The grounds for divorce in Washington are that there are irreconcilable differences. 
Not only do you not have to show fault, but fault is not even part of the equation. As a practical matter, it is not possible to contest an assertion that there are irreconcilable differences; protesting that the differences are reconcilable tends to prove they are actually irreconcilable. It is important to take the time to think through whether divorce is right for your situation. Getting good counseling, especially before things get too difficult, can help couples transform their relationship so it works. Counseling can make a difference even when things seem hopeless.