***** NOT A TEACHER *****
Good morning, Paul.
(1) You have asked a very good question.
(2) No, it is not just you.
(3) Neither is it a slew of comma splices.
(4) I have checked my books and wish to report this:
(a) All the experts call for a comma after "however."
(b) Some experts , especially in the "old days," recommended a comma after "therefore."
(c) I believe that most experts nowadays do not recommend a comma. In fact, some almost prohibit it.
(d) Many books are very kind: they say that it is often a personal choice.
(e) Thus, if you use a comma after "therefore," nobody can say you are "wrong"'; on the other hand, if you do not use a comma, no one can say you are wrong.
(f) I think I may be able to explain why many experts suggest NO comma.
(i) Say this sentence aloud: I want to learn English; however, I am too lazy to study.
(a) Do you notice how you almost WANT to pause after "however"?
(ii) Now say this aloud: I want to learn English; therefore I visit this website every day.
(a) Did you notice how you did NOT want to stop after "therefore"?
(i) The books explain that the word "therefore" still maintains its "adverbial force." It is not just doing the work of a conjunction. It is really working as an adverb to modify "visit." ("However" doesn't have this power. It is more like "but" -- only a conjunction.)