There are five reasons you can give for the marriage breaking down - unreasonable behaviour, adultery, two years separation with consent, 5 years separation without consent and desertion. Desertion is rarely used because it is difficult the criteria is difficult to meet. With adultery there needs to be proof of sexual intercourse with a member of the other sex which is difficult to prove unless there is an admission. Many people don't want or can't afford to wait 2 or 5 years.
Almost any behaviour can be unreasonable in some circumstances. Examples are spending too much time working/on hobbies/watching TV/at the pub, not spending time together, not doing any chores, financial irresponsibility, an inappropriate relationship with someone else, embarrassing or controlling behaviour. 5 or 6 reasons are required and it's is best to frame them in a non-inflammatory way so the divorce can progress quickly.
Everything I have written above applies in England & Wales. It would be different in Scotland.