The English language uses the continuous or progressive construction to express that an activity or situation is ongoing in time. Here are some examples:
I’m singing in the rain.
She was waiting for a bus.
The first sentence expresses that the singing is ongoing now (present progressive), and likely to continue for a while, whereas the second indicates that the waiting was happening over a period of time in the past (past progressive).
What is interesting is that the progressive construction saw a meteoric rise in use in the 19th century. This was in part due to the fact that English had no way of expressing a passive progressive such as The house was being painted. Instead, the so-called passival was used, as in The house was painting. It’s hard to believe today, but the introduction of the passive progressive was widely frowned upon, indeed detested, by some writers. R. Grant White writing as late as 1871 regards the combination is being as “an absurdity … monstrous … ridiculous”. He goes on to say that:
In fact, it means nothing, and is the most incongruous usage of words and ideas that ever attained respectable usage in any civilised language
Despite numerous similar attacks on the construction, it increased in use.
Another reason for this was that other constructions which involve the progressive were being used more and more, for example the so-called progressive futurate, as in I’m playing football in the park tomorrow. English speakers use this construction when they wish to express that some activity is planned or scheduled.
Recent research has shown that the progressive has continued to increase in use in the twentieth century. Linguists Christian Mair and Geoffrey Leech have shown that in written British and American English between the early 1960s and the early 1990s the progressive increased by 18.2% and 11.8%, respectively. 
In spoken English over the same period there is also an increase in the use of the progressive, but it is much less pronounced, as subsequent research has shown. This may suggest that the increase is leveling out. 
Many people have noticed that the progressive is now often used with verbs that previously weren’t used in that construction, for example verbs of ‘thinking’ and ‘emotion’, e.g. understand, love and want:
I’m understanding what you are saying.
We’re all loving this weekend break in the sun.
She’s wanting to finish her class early.
This use was attested in the early twentieth century, well before a well-known fast food outlet started using it. Usage is uneven at the present time, and we can say that this is a good example of a change in progress. Older speakers tend not to like it, or use it perhaps only with certain verbs, whereas the younger generations use it much more. This means that in all likelihood it will be seen as normal much more widely very soon, in the same way that the passive progressive became accepted over time.
This blog post first appeared in January 2018 on Spread the Word (Oxford Learner’s Dictionaries).
 Christian Mair and Geoffrey Leech (2006) ‘Current Changes in English Syntax.’ In: Bas Aarts and April McMahon (eds.) The Handbook of English Linguistics, Malden, MA: Blackwell.
 Bas Aarts, Joanne Close and Sean Wallis (2010) ‘Recent changes in the use of the progressive construction in English’. In: Bert Cappelle and Naoaki Wada (eds.) Distinctions in English grammar, offered to Renaat Declerck. Tokyo: Kaitakusha. 148-167.