The total and absolute truth about the reasons for divorce

The official set of reasons for divorce as laid out in divorce law are:
Adultery
Unreasonable behaviour
Desertion
Two years' separation with consent
Five years' separation without consent

The legal formality is that divorce is based upon the ‘irretrievable breakdown of the marriage.'  The very fact that you have taken steps to petition for divorce is enough to demonstrate that the marriage has irretrievably broken down.  It's best to have a solicitor to do this for you, although you can do it yourself.   

Go for the number one choice of reasons for divorce - Unreasonable Behaviour
Now to be honest you do actually need a reason but don't worry, just say the grounds for divorce are ‘unreasonable behaviour.'  This is the one most people use because it's easy and it has become acceptable to say anything you like about your wife's/husband's behaviour.  It does not have to be really bad, just behaviour that is a bit more than annoying will probably do it.  The truth or otherwise of your allegations is never questioned let alone tested.

Divorces based on the last three grounds are not used that much, in practice most divorces these days are based either on ‘unreasonable behaviour' or adultery and the great majority on ‘unreasonable behaviour.'  Neither of these two reasons for divorce involve the wait associated with the others, so they also have the merit of taking less time.

Adultery - probably not a good choice
Adultery is difficult and involves all sorts of stuff that you might not want to get into.  For men there is always the possibility of the President Clinton defence.  "I did not have sex with that woman."  When we all know what the lovely Monica Lewinski got up to underneath his desk in the Oval Office.  Best not to go there.

Don't contest the divorce, get ready for life after divorce
Don't waste your time, money and  energy on contesting the divorce, save that for the negotiations (or battles) to come on the real issues of the divorce, the children first and the money a close second.  A common reaction when you receive a divorce petition based on unreasonable behaviour is to say something like, "You've been just as unreasonable as me and so I'm going to defend it and issue my own petition."  

This is just not worth it.  Apart from perhaps making you feel a bit better its most likely effect is to prolong the divorce and increase the cost.  The quicker and less difficult the divorce the sooner you can come to terms with the reality and enjoy your new life after divorce.