Having never been married, I can only say what I have seen in my friend's marriages. For men or women, they don't make the effort to keep the relationship going.
Some take it for granted after a time. Others are too busy with life, that they forget the little things that made things special.
For some, it seems like the grass is greener in the next house.
A man might say he goes to another woman to find attention, but doesn't always realize all the attention and effort his wife puts forth in making his house a home. Or vice versa.