Preachers who make a habit of yelling do so for the same reason that some squad leaders/coaches/parents consistently yell while attempting to enforce discipline - they are hoping to make up for a lack of both thoughtful discourse and respect through increase of volume.

I don't believe that I have ever envisioned the preaching of Christ or the Apostles as including the practice of yelling, even when reading their most serious words (I don't know that Christ's throwing out of the money changers qualifies as a sermon). Possibly the only one who might fit that imagery for me would be John the baptizer.

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Soli Deo Gloria