Well they probably have a reason. If you check F3, you will likely see that you are weak compared to them. If that is the case and you are playing at maybe emperor and surely higher levels, that is is an invitation to either a demand or an attack.
The less open land still available the more I would expect an attack. If it is into the middle ages, I would want to spend resources on troops, not gifts.
I would give into a demand, if I felt it dangerous not to do it. I also give in to a demand for a lux, IF I expect to attack them in a few turns, as it cost me little and buys me time, rep has to be shot already though.
Sometimes you can be the big dog and still get the scenaro you mentioned. This tends to be at least after the late middle ages. Now they are mad as you had sometings they want or need.
In this case you probably are not worried and have items to get others to come in on your side, if needed.