Trading

After dealing with a number of Civ's, why do they ask for ridiculous trades ?

If I want a lux, such as wine, why do they want 3 or 4 of my lux's plus gold, and refuse to take less ?

I've all but given up on trying to trade lux for lux and now just sell my additional lux's. Am I missing something ?

If they are friendly they will usually trade one for one. But that might not be their first offer.

No matter how friendly they may be, they won't trade one-for-one if you are asking for their only remaining copy of a given luxury. So, when you browse the Diplo Overview screen, you should be looking for civs that have multiples of a luxury you want. If friendly, they will trade one-for-one, but they may not if they are sufficiently grumpy (often a civ will be nominally friendly, but inwardly irritated--maybe you entered into a DoF with their hated enemy, snagged a wonder they coveted, or settled a new city in their face).