C-fos appears to be important in effectuating morphological changes in the brain (specifically, increased dendritic branching and dendritic spine density) as the process of addiction occurs, as well as in overcoming addiction after the object of the addiction has been removed (http://www.jneurosci.org/content/26/51/13287.full).
In a recent post, I cited a paper indicating that an accumulation of ΔFosB can be considered the molecular "on-switch" of addiction in the brain. A 2008 rat study found that ΔFosB represses c-fos (http://www.jneurosci.org/content/28/29/7344.full). Anhedonia--an inability to experience pleasure from positive life events--is more common in psychoactive substance-using populations, and it lessens as they get off drugs (https://www.ncbi.nlm.nih.gov/pubmed/24270310). It appears as though the brain could be homeostatically acting to remain "faithful" to the source of pleasure it became addicted to and so represses c-fos production that would allow formation of competing neural pathways associated with other pleasures. That would help explain the inability to find much pleasure outside of one's addiction and perhaps the occurrence of monogamy. Again, very unromantic.
I feel like I should repost this all on February 14th to be contrary. "Your mate might be faithful to you just because of an accumulation of ΔFosB that prevents him/her from finding the same pleasure with someone else." That won't show up on a Hallmark card anytime soon.... Still, you might as well face it if you are indeed addicted to love.