Question: I usually freeze my leftovers.

Then, when I reheat them, the flavors are diminished - less salt, less chili, less everything.

Any ideas why?

Answer: 
There are a variety of factors at work here:

1. Freezing foods "improperly" (i.e. not flash-frozen, not vacuum-sealed) causes ice crystals to form within the food, damaging the molecular structures. This is what causes many frozen leftovers to become "mushy" or change in texture.
2. Again due to the formation of ice and the movement of water when the food is reheated, tiny particles such as spices can be lost in steam and/or runoff water.
3. Extreme temperatures (both hot and cold) can denature enzymes in the food, changing their flavor, texture, etc.
4. As food sits, flavors in the food may blend together in different ways, causing the food to have less distinct flavors.
5. If your freezer isn't especially clean and your food not well-sealed, your food may be absorbing other odors which are again "masking" the original flavor of your food.

Hope that gives you some ideas.
I asked this to a Swedish food guru and got the explication that freezing and reheating often softens the food. The effect is that each food piece has less time in the mouth before swallowing and therefore has less time to be tasted.

Not fully convinced but it might be worth experimenting with.