The type of love you are obviously focusing in on in your comment/question, is that of eros. (I'll call this intense eros to keep it distinct from general eros) The more realistic definition of this love, is that of attraction to the object of the sexual lean of ones brain makeup and operation. As there is the average male mental make-up, there is the female; and then there are points on either side of the bell curve, homosexual to asexual.
While the stimulation that the brain works up over chemical and firing processes which cause what is often called 'being in love' is sustained, it has been shown that there is a tendency to focus on the one object of that 'love'--for both male and female brains regardless of sexual orientation. Now, while we are 'drunk on love', having fallen head over heels with that person who is the object there is no problem...we are walking through open fields of soft, fresh flowers on Spring days--not a care in the world other than being with, thinking of and longing for being thought of by the one we are in love with. Most unfortunately, however, that state simply does not last forever--in almost all cases--three years or so are about the limit.
With marriage, a social arrangement that somewhat forces the nature in us to adapt to what is otherwise unnatural, had historically been thought of as an everlasting union. Therefore from the old days, and more strongly in some social structures today, a civil union is established on the assumption that the two parties involving themselves will stay together...not divorce.1 What that means, after the intense eros wears off--which it most certainly will--and the 'accustomed to' stage has well matured, (Cinderella and the prince have been worn quite thin) is that a couple will both have to work hard at trying to keep the embers of love going.
We have agape--the open and respectful love--which one can feel for all humans and animals and things, but that alone, will not make a good marriage. We have philos--the friendly, brotherly-sisterly love of companionship--but that alone, or even with agape, may not, in many cases, be enough to keep the marriage bonds strong against possible natural influxes in eros general...the man (or woman) can be forced by nature to urge to mate. It is surely better to work at maintaining agape, philos and eros in a marriage. Both partners have to equally work at it. Both partners should, as much as possible, see their mate as human equal firstly, then as the sexual being that they are, and keeping that in mind work together so as to help keep the balance. (some people have more, some less drive, some a larger hypothalamus, some a smaller one--the hypothalamus is the sex center)
What bothers me, is that so much of this information and study should be applied before going into marriage, but it isn't. Parents should be more open to help their children learn about sexual related matters as well as about the workings of nature, as well as giving them room to test and challenge social structures which tend to deny or shun these matters.
So in conclusion (and skipping a lot of data to show it) no, love in terms of that which occurs for the natural purpose of propagation does not last forever, but a couple can work hard together to at least keep the embers warm the fragrances in their active memory. And no, a mating partner was not originally a forever matter either, just as for all animals, so the married state can only be sustained by careful work before marriage and during marriage to maintain a working balance between the sexes (in most cases) and the individual humans.
1. In some contries, of course, a man can marry a given number of women, in at least one that I know of, a woman can have several husbands (I think that man's brothers?) In some Shiite Muslem lands, marriage can be temporary.