It’s so very common I’m sorry to say and by the time couples are seeing me the cat is out of the bag and the relationship is in crises. It’s extremely sad to experience the pain individuals go thru. It’s never going to stop, there will always be people who have affairs.
Why you ask?Top Mediation Tips

Little Emotional Connection: The common reason I see is the lack of emotional connection. When you marry and start your life together your deeply in love and focused on each other. Your partner is a priority and then other areas of your life start to take your focus. Perhaps a high-pressure job which takes a lot of your energy. Then throw children into the mix and the relationship totally changes for both partners, you are no longer priority to each other. Welcome to reality and living life in the 21st century.

Different interests: Sometimes partners develop interests and hobbies in different directions which of course is going to happen particularly in a long-term relationship. It’s very important to have different interests however when you don’t share your inner worlds with each other you may feel separated. This may lead to feeling like your partner isn’t interested and finding someone else.

Not Feeling It: Sometimes people fall out of love, certainly not by your own choosing it’s just something that can happen over time. There are many times in a marriage when one spouse will be more in love than the other. Busy lives can lead us to not have enough time for each other. Problems occur when the physical relationship begins to change and if one partner needs sex to feel loved and the other is too busy and not feeling it, big problems are ahead.

Boredom: A partner sometimes just gets bored. Maybe he/she is not working and at home with the kids. This might be satisfying in some respects but in others, you might feel extremely lonely and extremely bored with your life. Married couples sometimes know each other too well and there are no more surprises. A husband perhaps craving excitement and adventure in his life and if he can’t find that at home, he may find it elsewhere.

Resentment /Pay Back: Sometimes a partner might cheat purely out of vengeance. The other partner might have cheated on you and you’re simply wanting to get them back. A partner might be angry about not getting enough affection and will have an affair to get the other partners attention. A partner may feel overwhelmed in his life and an affair is a break from his reality.

All of the Above: You might cheat for any of the above reasons and might be a mixture of several issues. The reasons are never simple and never the same for each person. There is also endless other reasons which I haven’t mentioned from being with a controlling partner to feeling like you’re a parent in the relationship.

If you are someone who has strayed, and you wish to save your marriage (or at least see if you can), take a good look at what your relationship has become lately. Learning to avoid the things that commonly lead to an affair is your first step in rebuilding trust, moving on and preventing it from happening again.

I can tell you first hand witnessing the pain my clients go thru its devasting for all involved. Do some individual counselling to self-reflect and see what’s going on for yourself. Then continue with marriage counselling. End the marriage if need be but having an affair is not any easy road to travel down.