Faith

Love. What is there in love that one cannot find in any one friend?

Without trust there is no love.

So why cannot we love one's best friend?

Because that's just now how it works. Being "in love" and LOVING someone is totally different.
I think that to be "in love" with a person, you'd be willing to give up personal comfort for them to make sure that they are happy. That you would be willing to give up certain things, well, not necessarily "give up" so much as chill back on. Such as smoking, drinking, partying, etc. if your partner or lover was bothered by such things. Also, I find it difficult to believe that any one person who believes in monogamy would say that they trust their partner enough to believe that while they are out drinking or becoming intoxicated in any way that they wouldn't be even slightly tempted to do something devious with other people.
That being said, if one would happen to find out about another person coming into the what should have been dynamic relationship, creating a trifecta of sorts... That, I believe, would be grounds enough not to trust. That the trust would be broken by the sheer fact that the love has been distributed between more than just the beginning pair. There shouldn't be that type of problem if someones "in love" with you. That shouldn't even be an issue. But then again...

That's only my opinion.

And regardless of how much thought is put into actions, anyone can see that it's wrong to hang on another person while in an exclusive relationship. Just saying.