Of all the women in the world, why would your husband choose to cheat on you with your best friend? In the same breath, why would your best friend choose to betray you by hooking up with your husband? Is there such a big shortage of possible sexual partners that they'd rather pursue this option than look somewhere else.
Many people caught in this situation claim that theirs was not an intentional act. Their constant exposure to each other by virtue of their relationship to the betrayed one plays a big factor in the development of the affair. Their togetherness is seldom questioned unless there are obvious reasons to arouse doubts and suspicions.
It is true that affairs of this nature are not typically planned as most develop in time due to familiarity. It is safe to presume that the cheating parties may have tried to resist the temptation initially because of their relationship as a husband and as a friend respectively to the unknowing wife. An affair however is a clear testament to their failure to keep themselves in check.
You should not be wondering why the discovery of your husband's and best friend's affair is so much more hurtful that what you imagined infidelity would be. The affair involves two people who your trust with your innermost being . They are probably the last people you thought would hurt you this way. Your spouse and your best friend are supposed to defend you and support you and not connive with each other to deceive you. There is nothing bad about unconditional trust but the strangest things do happen including an affair which you never thought possible. Be aware of what is happening around you especially between the people closest to you.