I'm surprised spiro's haven't chime in.
My first reaction reading the question was: there's no "for whatever reason". Not once have I wondered "why" on my encounters with sharks.
Granted I've never done a "shark dive". But I had encounters with sharks. Maybe a couple of dozen close encounters and only 2 were tricky.
Both with Bulls, one come to me when I had a fish on a stringer as I was coming up, I saw him coming and it was obvious he wanted the fish. I was very aware is bad form to let them get your catch n you're supposed to punch him in the nose but somehow at the moment my brain rejected that tactic as the most stupest thing to do. So without much ceremony I gave up the fish. Was a good thing too, because 3 more came within my view right away. They just went after the fish and that was that.
The second time I was on a wreck with my husband and other people and another bull started circling us, again the behavior was obvious, he looked pissy. My husband and I exchanged the universal sign for WTF, got closer together and got ready for whatever I held my speargun with the butt forward instead of the spear figure. He must've sensed that we also got pissy, because he decided to bother other divers.
The rest of my encounters have been great with some of them qualifying as magical.
So my suggestion if you want to be prepared, do the same with sharks as with humans:. Be respectful but neutral while getting ready to either smile or disable.
There's no guarantee that you'll be successful disabling either one, but you'll go down fighting.