Speakers tend to use "so" and "very" synonymously:
They were very brave.
They were so brave.
Speakers also use "so very" to express intensity:
They were so very brave.
Now that's so very cool, wouldn't you agree?so as intensive.
People sometimes object to the use of so as an intensive meaning “to a great degree or extent”:
We were so relieved to learn that the deadline had been extended.
This usage is most common in informal contexts, perhaps because unlike very, it presumes that the listener or reader will be sympathetic with the speaker’s evaluation of the situation.
Thus you would be more likely to say
It was so unfair of them not to invite you
than to say
It was so fortunate that I didn’t have to put up with your company.
For just this reason, you can sometimes put intensive so to good use in more formal contexts to invite the reader to take the point of view of the speaker or subject:
The request seemed to her to be quite reasonable; it was so unfair of the manager to refuse.
Just remember not to overdo it.