Based on what I read on Business Insider (weirdly) (url: http://www.businessinsider.sg/why-we-cheat-on-people-we-love-2015-4/ ) I have gain this conclusion, which I personally based on past experience, can fully verify.

For the ultimate fulfillment between a couple, there is 3 aspect that needs to be fulfilled:

Sex, Love and Attachment.

The failure of any of the above 3 may drive a person into cheating or quitting the relationship.

Sex – Sex is simply a basic human biological need. Sex drive exist in every one, and that it is like an “intolerable neural itch” that gets people to seek out candidates to pass on the genes (guys) or the urge to get the genes from the most eligible candidate (girls).

Romantic Love – Its scientifically proven that romantic love (lets call it “love” in short) operates as a motivation and reward system in the brain. Its not an emotion, but a drive; the brain reacts to the person you love in the same way a person take drugs or has an orgasm. Meaning, love evokes a chemical reaction in the brain. Thus love can become dull, and people start contemplating break ups, because they no longer “feel the love” from their partner. Which means, both individuals in a relationship have to make an effort to continually “reward” one another constantly; constant reinforcement and reassurance to one another of their love for each other. Complacency to do so will cause the other party to feel neglect and feel unloved; thinking that you love the other person but not expressing it, is utterly meaningless. Love is NOT an emotion, thus, if you do not express your love, motivate/reward your partner’s love – it will cause quarrels and arguments which you will not be able dispel. So never stop loving, and never stop showing your love for your partner.

Attachment Love – Attachment Love (lets just call it “attachment”) is the calm and security we feel with a partner. This is what people always tell their partners that they feel at ease and comfortable, or even happy when they are with their lover. When you get deep attachment with your partner, you stop quarreling for most cases; even in the absence of love. And when this attachment exist deep enough or with enough time passed, it enables the couple to raise children as a team. Attachment IS perhaps the primary reason most couples in long relationship ties the knot and get married (even if they are no longer THAT romantically in love). Attachment alone IS possible keep a couple together, to get married and raise children – with the absence of Love. But a relationship without love would mean that a person could feel deep attachment to their partner, but at the same time, feel intense romantic love towards someone else, and even feel sexual attraction towards another person (read: cheating).

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Love however is not Sex. You can LOVE without sex. But if there is no sex, the person may be driven to satisfy their sexual urges elsewhere. Love and Attachment but no Sex (or poor sex) will cause the same thing. Sex is not Love either. Human beings are perfectly capable detaching sex from love (both genders); or even looking for multiple sex partners to satisfy their sexual hunger. So in essence : THE LACK OF SEX, will definitely result in the partner seeking the sexual gratification elsewhere.

Attachment with Sex is basically what most married couples are at. Many may have their romantic love dulled, but there are together, and continues to satisfy each other’s sexual needs and the sex will help to reinforce the attachment and the illusion of romantic love.

Love + Sex is usually the driver for 3rd party relationship, or the cheating cases. The excitement and exhilaration that comes with “cheating” lights up the brain like a drug, thus people just can’t stop themselves from diving deeper into the rabbit hole. But the lack of attachment would mean that, the “promise” of divorcing their spouse to be with their new love usually fail to come true. And for those which went thru with the divorce, tend to stumble upon a new spouse which they are not “attached” to and the relationship turns into tragedy.

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Love + Sex + Attachment = The perfect relationship. And this should be what everyone should work towards for. However, this is not a guarantee for no cheating, as sexual lure exists all the time, and that it will take discipline in the individual to avoid cheating in a scenario that they already have the perfect relationship.

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edit – 13 august 2015

Watch this talk by Helen Fisher (who came up with the theory above)