It's the standard rule you can find in general grammar books.
...It is not a 'rule' that has to be followed in every case.
. But still, when you say
I'm loving this book, you refer to the enjoyment you are experiencing while reading it and you could easily substitute "enjoying",
Thst is not to say that 'loving' means 'enjoying
whereas when you say
I love this book you cannot so easily substitute
enjoy for
love, save for more specific cases, e.g. you know the book well and often re-read it.
i.e., you can substitute 'enjoy' sometimes.[...]
I love hamburgers / ghost stories (in general, it's a permanent fact)
while I might perfectly (but not necessarily) say
I'm loving this hamburger / ghost story (that I'm eating / reading now)
That is the point I made with 'limited duration'
[...]Also,
I love this picture but never
I'm loving this picture unless it were so big that you hadn't finished looking at it while you said this.
I have just borrowed a picture from my friend. It will hang in my sitting room for a month. I am loving this picture.
And by the way,[...] there's a long (24 replies) old (May 2009) thread on this, right at the botton in the similar threads section
Note the post from Barb_D:http://www.usingenglish.com/forum/as...tml#post310408.