"Because when you start CRUSHING "

I think that "crushing" or that infatuation people feel for the first few months of a relationship is just natures way of making sure that two people stay together long enough in order to procreate.

Many women after some years of being in a relationship complain of "boredom" and it's important for men to understand that when she says this in reference to your relationship, take her very, very seriously. This is the time to ask yourself if you are spending enough quality time together or if you are taking her for granted.

What she means more of less is "I am bored with you". It seems that women attribute boredom in their relationship with there being something wrong with the relationship. She becomes vulnerable to the next guy who promises her the world, and there is no shortage of them.