That's easy! When a person isn't completely satisfied with their marriage, they have three options. The first and best option is to seek professional help and work together to save the marriage. The second option is to get a divorce so that they can find someone new to love. The final option is to cheat.<br />
Not everyone cheats, but of those who do, they do so for various reasons. Cowardice and selfishness are the short answers. Some will claim that they didn't want to hurt their spouses feelings by telling them that they aren't happy anymore. It's really because the cheating spouse is too cowardly to handle the drama when they reveal this to their spouse.<br />
Selfishness is when the cheating spouse enjoys everything else in the marriage (nice house, steady income, plenty to eat, money to spend, and such). They want attention and affection from someone else but they don't want to give up the nice things that they have in their marriage. A cheater like this is using their spouse. This is why cheating is bad and why it's best to avoid dating or marrying a former cheater. I won't go so far as to say once a cheater, always a cheater, but if this person cheated once before, they may do it again when they become unhappy with you. It's a risk you take.
I think it is a passive-aggressive move for cowards too afraid to face their problems head on. If you are in a relationship, work on it or get out, but be honest. There are the options of counseling, communicating or even a trial separation to work out the finances, the splitting apart of basically a lifetime together. Cheating essentially allows the cheater to not be alone, have a supporter and have his (her) ego stroked at the same time. Alone they would have the courage for none of it. Sometimes it is a control move to get the spouse to get in back in line. (If you don't do what I tell you, this person will). The funny thing is that many times the cheaters are people that verbally pride themselves for being loners, independent and not needing anyone, but in the end they are insecure, scared, immature children.
The real question is how did this mate get away with it for the first 35 years? In other words, it probably wasn't the first time.
The comments are too trite or simplistic. Sex is only one part of married life. Physical changes, illness or injuries, to name a few, come into play. One very real part of that is a change in hormonal levels. The wife may have excellent reasons for not taking hormonal therapy, such as a history of cancer. That does not change other parts of their relationship. Cheating can and is rationalized in many different ways. Sometimes it is the lesser of many evils.
If a woman feels neglected then she will seek another man to offer her attention. If the man is attentive and she still cheats then she is just being disloyal or maybe reacting badly to something she's going through or she has fallen out of love. Some women find attraction from other men to be a validation process, they are validating for her that she is still attractive and desirable and this goes for men as well. Perhaps their wife doesn't want to be with them sexually anymore so he goes out and gets it someplace else. Either way I consider it disloyalty. If one party wants someone else then have the decency to leave first and then hook up.
They want to recapture their youth
they changed their mind about the one they used to love.