I'm fine with the way diplomacy is right now, except for the fact that modifiers appear to be permanet and never disapere. This means that if a civ turns guarded on turn 100 say goodbye to any friendly relations. If relations could fluctuate then diplomacy would be much more enjoyable
They can. I've had civs go from hostile to friendly and stay there. Usually happens if political situations change, like you finally denounce their worst enemy etc.
To the OP: Remember, its almost impossible to be liked by everyone. If they don't want your help/support...on their own head be it