To understand the difference between “in love” and the “the fog” one needs to understand emotions and what love is to some extent. Love is the feeling or feelings produced in reaction to a chemical c0cktail and connections in the brain created in response to external stimuli by association.
In short your brain responds to people & things by creating neural connections that are mediated by specific neurochemicals. Thus when you are reminded of or see that person your brain circuits fire the recognition and produce those feelings. Those feelings are emotions and are a result of the associations previously created. The emotion of love is closely tied with the reward systems of the brain which are also identified with addiction. Addictions produce a strong desire for more and serve to replace other neural connections to other people and things.
Much has been learned about the neurological basis for love in the past dozen years but it is still not fully understood. I have included a link to a review article below for those inclined to learn more. Even though emotions and the basis for them are not fully understood, they are understood well enough to create the generalizations above. These generalizations are enough to explain a significant amount of the emotions encountered in a daily reading of TAM.
Two different aspects of love have been mentioned in this thread, infatuation and long term (true) love. It has been shown that infatuation is linked to the euphoria, eroticism and excitement chemicals (see quote below) whereas long term love is associated with more nurturance, protectiveness, low stress. Two commonly distinct types of love can be understood as extremely different mixtures of the same chemicals. For each individual that you have love for will probably have a slightly different mixture of the chemical c0cktail. For instance family may be high on nurturing and so on. The point is that all types of love from romantic to agape are manifestations of a different mixture of the c0cktail. It’s basically the same chemical bath for everyone you love.
When you recognize this, it also explains that it is possible to be romantically in love with two people at the same time. It is not probable though because several of the neurochemicals including oxytocin actively break neural connections with others. Oxytocin is released via sex through orgasm and is responsible for the nurturing long term bonding components of love.
Now back to the question about the difference between “in love” and “the fog”. If you are married and you are still in love then you are probably at the long term true love spectrum of the chemical mixture. You probably (but not necessarily) went through an infatuation stage with your spouse. However since you may not have been attached to anyone else your new addiction to your pre-spouse may have gone unnoticed except by friends that saw less and less of you because you were then head over heels in love. Friends understand that is what happens often.
Now suppose you are married and in long term love but begin to have feelings for another. The addiction components of the c0cktail propel you to escalate contact as much as possible. Soon you get the rush and excitement and you are infatuated when the other. The escalation of contact from the desire to be with the other leads to a reprioritization of what is important. These new priorities are at odds with the marriage and don’t make sense. Furthermore when the oxytocin kicks in and neural connections with the spouse are replaced with the other then links to those old feelings diminish and disappear. This is often noted as rewriting history of the relationship because the way you remember things has changed and is changing. This is the fog. It is noticed more because it is in stark contrast to an existing relationship. Now if you have lost the love for your spouse you will still go through the steps above but your connections to your spouse have already been lost through other means and it is still the fog.
A review paper on emotion can be found at
The brain basis of emotion: A meta-analytic review, BEHAVIORAL AND BRAIN SCIENCES (2012) 35, 121–202, Lindquist et. al. http://nmr.mgh.harvard.edu/~lindqukr...al_BBS2012.pdf
That above link is stale, see: http://www.ncbi.nlm.nih.gov/pubmed/22617651 http://www.unc.edu/~kal29/docs/Lindq...al_BBS2012.pdf
A good excerpt that shows the chemicals involved is:
Similarly, love is arguably a primary affect at the ecological level, associated with intimate displays (contact comfort, pheromones). The subjective experiences associated with love may be associated with a neurochemical c0cktail including high endorphins (euphoria), gonadotropinreleasing hormone (GnRH: eroticism), dopamine (DA: excitement), oxytocin (OXY: nurturance); vasopressin (AVP: protectiveness), CRH (stress); and low serotonin (5-HT: submission) (see Ortigue et al. 2010; Panksepp 1998). Understanding of these systems, their evolutionary and genetic bases, their complex interrelationships, and their communicative role in social interaction is proceeding apace. - Ross W. Buck, Prime elements of subjectively experienced feelings and desires: Imaging the emotional ****tail