I found this to be very helpful in understanding St. Paul's sometimes complicated writing style. This is from The Navarre Bible commentary on Romans & Galatians (pg. 54-55).
"...[the
epistles of Paul] have all the features of real correspondence; one
realizes immediately that these are not letters written with a literary
purpose, nor are they intellectual reflections cast in letter form
(quite a common practice in all languages), nor elaborations of some
earlier text. They have all the immediacy of things said or dictated on
the spot. St. Paul links ideas by association, he goes back a number
of times over things he has already spoken about and keeps repeating
ideas which he sees as very important; he develops his arguments slowly,
moving in concentric circles; sometimes he rises suddenly to dizzying
heights, pursuing an insight; on other occasions he writes straight from
the heart and the tone is warm and passionate; he sometimes uses irony,
reproach and even cutting words if he feels that is what his readers
need.
Paul's style does not follow any fixed method;
but there is a basic order to it. In each letter there are one or more
core themes, to which the Apostle keeps returning, and these lead him on
to other ideas which seem to be incidental. This explains why we sometimes find unfinished sentences: the writer's thought has gone off on a tangent and does not come back, but it is easy to see what he is saying: in fact, these unfinished sentences have a poetry and strength and vitality of their own.
Still, these epistles largely do keep to the general structure of a traditional letter. Paul begins with a greeting, which is not a stereotyped form of words but a vibrant, supernatural recollection. He gives a little news about himself and sends his best wishes--for peace, grace and divine assistance. He then moves on to deal with the matter(s) he wants to expound. He reminds his readers of their duty, in all situations, to be charitable, and, always using new imagery, he paints an attractive picture of what the Christian life involves--how magnificent it is, and how demanding it is also. He ends the letter renewing his good wishes and greetings, and praising God."
