How you come to know that your closest friend is cheating on you?

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I had always been intrigued how a person known as closest friend could start cheating. However, experiences and discussion which I read in mylot clearly indicate that such thing is really taking place. It is not that I do not know about it. I have heard number of such stories circulating that girl or boy friend was also going with other or was seen in intimate position other than his/her partner. How such thing is taking place? Is it not that it was never a close relation at all and was only limited to, what we call, infatuation? What could be reasons for such cheating? Would you please educate me on this matter?