If you were to really dispassionately analyze the transaction, lending something to a friend or acquaintance is predicated on there being enough social capital at risk in the relationship (or shared network) to insure things go well, or that you're made whole if they don't, along with a general knowledge their ability to cover potential loss. IMO codifying that into a financial relationship signals that you don't trust that reservoir of social capital, and in doing so erodes it. "Oh, I guess he doesn't actually trust me that much, if he's asking me to post collateral to borrow his truck".