I couldn't resist replying Darth.  I asked this question many times and many years ago.  A chemist friend and fellow curry head believed it was to do the the chemical breakdown that occurs when it cools/freezes and is the heated up again (curries are a chemical process).  For example when oil is passed through and superheated in a cracking plant (refinery) it produces several new products like petrol diesel and paraffin, none of which smell like each other or the original oil.  There are also various enzymes on the loose in curries, some good an some bad, and their function is to break down food and create conditions for bacterial growth which completes the decomposition process.  Smell is a gas laden with molecules which we are sensitive to, the process described above also affects this.  I know there are a few professionals in the chemistry/physics fields lurking so I expect we shall have some more informed answers Darth.  Perhaps you should ask Jeeves :

Check your email

Vee haff vays of making zee curries ya know  ;D
CP

Followed C P ' s lead and found this info Darth -
Most flavour molecules, that we detect with our receptors on our tongues, are fat soluble. During cooking, some of the molecules are dissolved into the fats and oils present in the food, but cooking is a relatively quick event given the size of some of these flavour molecules. Thermodynamics come to our aid in that given enough time most anything will mix themselves together, heat just speeds up the process of distribution. So your curry from yesterday tastes better as more of the flavour molecules leak out of their confines (perhaps the cells of the food item) into the fats and oils they are soaking in. Then your tongue becomes coated in the fats/oils where the taste sensors can come into contact with the flavour molecules, in a higher concentration than yesterday. This explains why on the whole, foods with the fat removed appear less flavourful unless the recipe is modified to counter the loss. The fats and oils also act to hold the taste to the tongue for longer than say, water soluble flavours. So there you go!

Heat and time also cause chemical changes, CC. In particular, a change away from the original taste.

I find that things heated up (not just curries) just don't have the same fresh taste, although the 'depth' (or complexity) may well have changed. I can honestly say that personally I have never had a curry which tastes as good as or better than the original when reheated the next day. It tastes OK, yes. But it tastes different.

At best, the spices might be more pronounced - and I wonder if that's what people are latching on to? But I find the garlic and fresh coriander tastes are less and there is usually a bitterness that wasn't there before (or is it a missing sweetness? I don't know).

This happens with curries from my favourite restaurants and with those I cook myself.

The other thing to consider is that since the curries we are all apparently looking for are cooked fresh, is the subject of reheating them the next day connected with finding the secret or not?

This makes me wonder if we are chasing the same "taste"
When I reheat a frozen curry with the "taste" the wonderfull aroma fills the kitchen.
If I reheat a curry from a takeaway, without the taste, it becimes very ordinairy and oily.

I suggest it may be something to do with aromatic ingredients. The best examples are perhaps black pepper (western cooking as well as Indian), fenugreek leaves, and a mix of aromatic ingredients often referred to as garam masala.

These are added at, or near to, the end of cooking, largely in order to preserve the aroma until the dish reaches the table. Otherwise, why not add all the ingredients at the beginning of the cooking process?

The longer the period since the aromatics are added, the lower the aroma.

Regards
George