In any long term relationship things pile up. Little frustrations, hurts, insensitive remarks can all add up over a period of time and each little thing is like a brick in a wall. After a while you will have one very big wall. That's the point where one party will throw their hands up and say 'Enough!" and leave. The important thing is to make sure your relationship never gets to that point.
One thing you have to do is un-learn much of what you've been told about women and what they want. If you think women are needy, and high maintenance you need to re-evaluate your thinking (of course some women are that way, but not most of them). While this may sound stupid here is something you should try: stop thinking of your woman as a woman, instead think of her as a person. Treat her the same way you like people to treat you.
One of the biggest mistakes men make is they either treat their woman like a whore, a surrogate mommy, or they go to the complete opposite side of the spectrum and put her on a pedestal where they treat her like she's made of porcelain. The best thing to do is treat her like a human being: a unique individual that has her own mind, needs, and wants. Get to know the real woman, not the person you want her to be, or think she should be.
Don't lump all women together. Don't assume your wife or girlfriend likes a certain thing or should act a certain way just because some other woman (or your mom) does. More than likely you have several male friends. Do you treat each one of them the exact same way, or do you modify your behavior (slightly) for each unique relationship you have?
Women leave men because they are unsatisfied in one way or another. Each relationship is different but at the end of the day your wife or girlfriend will leave because she is not getting enough of her needs met, it's as simple as that.
Most women want their husband or boyfriend to be a friend and confidant. They want someone who acts like they actually like them. Many men don't act like they even like their women and they certainly don't treat them like a trusted friend.
Women have sexual needs just like men. Our society has done a huge disservice to men and women because it tells men they have this huge sex drive and that it's okay to pick up women whenever they want to and that women have to be 'good girls' who will only have sex with their husband.
The fact of the matter is that is not true. Women have the same sex drive as men, they are just conditioned to not act on it whenever the whim hits them. What women don't want is to think that their man only wants them for sex. Women will often disconnect from their men physically because even though they like sex they also like non-sexual physical contact and many men think they have to turn every incident of physical contact into a sexual encounter. After a while your woman won't even want you to touch her, and she'll find someone else.
So if you want to avoid having your wife or girlfriend leave and you want to answer the question: "why women leave their husband or boy friends " just follow some of these simple tips and treat your women like someone who you like and love and your relationship should last a long time.