The adverb therefore should be used with caution, as it is often at the center of run-on sentences. Therefore is not conventionally considered a conjunction, so it cannot fuse two independent clauses into a single sentence the way conjunctions like and, but, and because can. For example, the following sentences are run-ons because they use therefore as a conjunction introducing an independent clause:
The players were my heroes, therefore, my dad was my hero because he was the coach. [transcribed in Houston Chronicle]
The European Central Bank and the EU itself will not jeopardize their existence, therefore they will do everything they can to maintain the euro and, therefore, Greek solvency. [Wall Street Journal]
However, most have less difficulty in choosing their network of preference, therefore it certainly becomes a little simpler to narrow down the choices. [Headline News]
There are a few ways to fix sentences like these. One, make the clause beginning with therefore a separate sentence:
The players were my heroes. Therefore, my dad was my hero because he was the coach.
The players were my heroes; therefore, my dad was my hero because he was the coach.
Or three, add a conjunction to link the two independent clauses:
The players were my heroes, and therefore my dad was my hero because he was the coach.
But when therefore is an adverb, it works perfectly well in the middle of a sentence—for example:
The roads in Amsterdam are more vacant and therefore the cab drivers can speed and manoeuvre along more easily. [Calgary Herald]
Cameron’s under pressure from the Tory right for being a softie and, therefore, promised tougher action on the tabloid bad guys. [Guardian]
So they discuss how they can position him to serve the maximum time, therefore keeping him off the streets in the future. [New York Daily News]