There are a number of definitions for concupiscence. My first professor of moral theology (which our school called Christian Ethics) did not like concupiscence to be reduced to sexual desire, or eros, with which he had no problem. He was very pro-eros, a word he expanded to mean our desire to get outside ourselves, to reach out to the rest of creation. He would probably deem a healthy interest in field grass erotic. An interest in field grass that consumed your life and soul, however (like killing people for their collections of rare field grasses) he would find an example of concupiscence.
Anyway, he took a Thomist attitude towards concupiscence, and although my notes are in a closet across the ocean, I suspect he said something like what I am going to write now, which is that concupiscence is wanting more than your fair share of something.
Having said that, I will hasten to say that very often people do not get their fair share, and so they have to fight for justice and get their fair share. But just as often, we are given something God has allocated, enough for our flourishing, and then we want more.
The image that comes to mind is a mother cutting up a chocolate cake for her children. The biggest, oldest child gets the biggest piece because he has the biggest appetite. The smallest child gets the smallest piece. The child with a slight allergy to strawberries does not get a piece with a strawberry on it. The child who has to wait an hour after digesting dinner before eating chocolate will get his cake later than the others, and so on.
Now, in a perfect world, the children would sit quietly at the table, trusting in their mother's ability to know what is best for them and to serve them their special piece of cake in due time. However, in my image, the children do not live in a perfect world but are infected with concupiscence and so long to fall upon the cake as soon as their mother's back is turned, and start munching away in great, greedy handfuls, even before they have finished their dinner.
To extend this analogy, I suppose the mother sometimes decides to pretend not to see this mad orgy of cake-eating, and allows her children to suffer the ill-effects of their own concupiscence. Sin, as Sister Wilfreda said back in Gr. 9 Religion, has its own built-in consequences. So the children become terribly ill, and we hope they have learned their lesson and do not increase their miseries by searching the larder for another cake to devour, hoping rather irrationally for a better outcome.
It is a very strange thing in human nature that we always want more. I was first struck by this as a teenager when a man I worked with described all the things he had bought and all the things he wanted to buy. I pointed out that he had a lot of stuff already, and he said, eyes twinkling and yet dead serious, "But I want MORE!"
When I was much younger, I was not satisfied with being admired by only one young man. I wanted lots and lots of young men to admire me, at least three. This is now, thank heavens, not such a concern. As long as my husband admires me, that is enough, and if other men do, too, then that is a nice bonus. It helps to be forty.
However, there are still other things that I want more and more of, definitely more than my fair share. Chocolate cream pie, for example. If I make a chocolate cream pie for a dinner party, I am usually left with half of it afterwards, and so I eat rather more of it than I should for breakfast and lunch. This is supremely irrational behaviour, so why do I do it?
I also enjoy more than my fair share of sleep, coffee, reading blogs and resting from housework. Also irrational. I blame concupiscence.
One thing I notice about some readers who write in is that they are positively longing to start romantic relationships with young men before the young men have given the obvious sign that they would like to be in a romantic relationship. I suffered from this myself for a decade or three, so I find it very interesting. Also interesting are the many excuses readers come up with to continue pursuing a man who has no obvious interest in them. I did that, too, and I wonder what that is all about. Is it like being determined to eat the wrong piece of cake?
You can guess where I'm going here. Could it be that, by chasing men, particularly those men who show not a whit of interest in them, really, women are manifesting a form of concupiscence? And could it be that, by lazily not bothering to get to know real women, but instead messing around with internet porn or solely-internet relationships, men are manifesting another (and worse) form of concupiscence? Both situations show irrational desire and the desire for more that one's allocated share of something.