Grounds for Divorce
In Illinois a party needs to prove one of several grounds or that the spouses have irreconcilable differences, subject to certain terms and conditions, in order to enable a judge to dissolve the bonds of marriage.
The more common types of grounds are (i) extreme and repeated acts of mental or physical cruelty (ii) adultery (iii) habitual (over two years) drunkenness (iv) excessive controlled substance (drug) use (v) desertion (over one year) and (vi) commission of a felony.
However, if both spouses agree that (i) their marriage has irretrievably broken down because of irreconcilable differences (ii) past attempts to reconcile their marriage have failed (iii) future attempts to reconcile would be impractical and (iv) they have lived separate and apart for the immediate past six months or more, then they may divorce based on irreconcilable differences. If only one spouse agrees that the parties' differences are irreconcilable, then the parties must live separate and apart continuously for two years. "Separate and apart" does not necessarily mean under separate roofs.
It is possible to live separate and apart under the same roof where the spouses no longer hold themselves out as a married couple (e.g. he does not cook for her, she does not do laundry for him, they do not spend vacations or holidays together, and their sexual relationship has ended, etc.).
There is really no economic advantage to alleging a particular type of ground, and typically no reason to make a public record of a private act that is not criminal. Therefore, irreconcilable differences is the most common reason alleged in a divorce case.