There are a couple of similar threads:

4 days on VLC - suddenly feels happy?

Low Carbs - Coincidence?

Some possible reasons could include:

• blood sugar regulation, so you don't have the bad moods that come with hypoglycemia
• better access to energy
• improvements in brain functioning

ETA:

Incidentally, while researching something else, I came upon this today (emphasis mine):

Low-carb diets, fasting and euphoria: Is there a link between ketosis and gamma-hydroxybutyrate (GHB)?

Abstract

Anecdotal evidence links the initial phase of fasting or a low-carbohydrate diet with feelings of well-being and mild euphoria. These feelings have often been attributed to ketosis, the production of ketone bodies which can replace glucose as an energy source for the brain. One of these ketone bodies, beta-hydroxybutyrate (BHB), is an isomer of the notorious drug of abuse, GHB (gamma-hydroxybutyrate). GHB is also of interest in relation to its potential as a treatment for alcohol and opiate dependence and narcolepsy-associated cataplexy. Here I hypothesize that, the mild euphoria often noted with fasting or low-carbohydrate diets may be due to shared actions of BHB and GHB on the brain. Specifically, I propose that BHB, like GHB, induces mild euphoria by being a weak partial agonist for GABA(B) receptors. I outline several approaches that would test the hypothesis, including receptor binding studies in cultured cells, perception studies in trained rodents, and psychometric testing and functional magnetic resonance imaging in humans. These and other studies investigating whether BHB and GHB share common effects on brain chemistry and mood are timely and warranted, especially when considering their structural similarities and the popularity of ketogenic diets and GHB as a drug of abuse.

So that's an acknowledgement of the phenomenon and another hypothesis of a mechanism.