Every circumstance is different. There appears to be 3 basic types of cheaters.
“The Incessant Cheater” - whose motto is “variety is the spice of life”. He/she loves to flirt and seduce. Most likely they have never been faithful in any long-term relationship.
“The Unbelievable Opportunity Cheater”. He/She was not actually looking to cheat but an "unbelievable opportunity" fell into their lap. The woman/man may be “out of their league” (beautiful, famous, rich/powerful) or someone they had a secret crush on. When the “once in a lifetime chance” presented itself in their mind to say “no” would have been the equivalent of throwing away a winning lottery ticket.
“The Discontented Cheater”. Of the 3 basic types of cheaters this is the one who actually feels “justified” on some level. They see their cheating as being the result of something their mate is not doing for them….(sexually, showing appreciation, respect, admiration, or they feel taken for granted. Bottom line in their mind they are “reacting” to how they are being treated. It's not difficult for a kind stranger to put a smile on their face or in their heart.
The first two types of cheaters cheated simply because they were in the mood. There is nothing their significant other or spouse did to cause them to stray.
The "Discontented Cheater" however also cheated by (choice). If they were that unhappy they could have chosen to end the relationship/marriage. Nevertheless the betrayed person in this particular situation may see that their neglect or behavior may have contributed to their mate looking outside of the relationship. As for the "other woman" or "other man" who knowingly becomes invovled with an attached person also bares some responsibilty for their actions which may have contributed to destroying a family.