Oh this is complex Wobbly - I'm even writing a book about this atm and I'm finding it hard to precis this bit.
People who have diagnosed or undiagnosed depression are much more vulnerable to an affair, because there is no experience like a new sexual adventure to mimic the feelings of being 'alive' with all the senses tingling. This has an even more devastating and apparently positive effect on the depressed than on someone whose emotional health is stable without highs and lows. For depressed people, the artificial 'high' of a new affair is like a 1000 volt life-charge rather than just the normal pleasurable adrenaline most people feel when are in the early stages of a new relationship.
But this is not to be confused with people who are just a 'bit down' with the way that life has panned out; not necessarily in their marriages, but in the way they look in middle age, their career progression, their material wealth, the quality of their friendships - all the things that make up a life. When people like this have an affair, the initial 'high' is often replaced by guilt, the stress of deceiving a much-loved partner, the constant nervousness and risk management about being found out, the stress of keeping two partners happy, the impossibility of feeling connected to the partner you're deceiving, the feelings that everything is going to come crashing down at any moment. Those feelings of 'deliciousness and wonder' have worn off to an extent and although the person might still be addicted to the affair, if they stopped and analysed it properly, they would acknowledge that something needs to give. Either they must make a decision between the two partners or the affair must end.
These feelings get acted out as stress, distancing, appearing troubled and unhappy, going into hiding, apparently taking no joy in anything anymore, an unwillingness to socialise or meet up with friends and family, irritability, aggression and moodiness. All of these behaviours are often associated with (and mis-diagnosed as) 'depression'. If a bewildered and loving partner queries what on earth is wrong, people who don't want their affairs discovered will often lie and say that they are depressed or stressed about work, money, elderly parents, bereavement - anything to put their partners off the scent and very manipulatively, so that they can be treated with a kindness that they absolutely don't deserve at that point. I've known people to be more angry about that more than the affair itself.