What are the causes of adultery?

The question isn't why other couples my be vulnerable, the question is why YOU may be vulnerable. There are as many reasons as there are people in relationships. It's not an exact science when it comes to figguring it out. First you have to find out what's wrong with the relationship, then you'll know how to fix it, if that's even possible.

Since we live in a time where people are very into instant gratification, there are a lot of people out there that enter relationships for the wrong reasons, or stay in them for the same. People can change, tastes change, desire fades, what's left over is what you have to base a relationship on and in our current me me me society it's not typicaly the same thing as wehn it started, and unfortunately not always enough.