I don't think attitude has much to do with it. I've found I'd get one sort of deal upon initial contact (when they're annoyed let's say) and then a worse deal just a few turns later when they're polite.
Some amazing revelations have come from research into game mechanics. I lack the intellect and the experience to do the research myself, but there's a great opportunity for someone to immortalize themselves if they could come up with the definitive work on the banking strategy. Of course it may not be researchable. Things like corruption, war weariness and goody huts are coded into the programming, so it was just a matter of experimentation to determine their mechanics (and some occassional help from Firaxis). The loaning of gold may be an unintended consequence of having a trading system so there may not be well defined mechanics about how it all works. Suffice it to say, whenever there is some possibility that you won't have to make all your payments, the deal makes sense under virtually any terms.