There’s almost always more than one reason for something. “Stay away from the river: you’ll get wet and catch your death of cold; you’ll drown; it’s full of hungry piranhas”. Any and all of those are possibilities. But at least there’s fun to be had in going there anyway. Then there are lorries: big things; slowish; difficult to see around; lots and lots of kinetic energy. No fun at all to be had in going there. But lots of reasons to avoid the things.
The most common training mantra regarding lorries (and other suchwhich) and the overtaking thereof, is to hang back, clear of the vehicle, until your exit beyond is clear: i.e. do not sit up the back of the slow car overtaking the slower lorry, but wait until it’s crawled past and (hopefully) pulled back left. Reasons are, I hope, bleedin’ obvious – Big vehicles have Big blind spots. And you’re on something very small and vulnerable, should the Big decide to go elsewhere than it’s current trajectory. So, keep clear until you can get past in one smooth, fast manoeuvre (both wheels on the ground, by preference).
Further to the above, if you’re on a single carriageway road, you’re minimising your exposure time to oncoming vehicles and, on any road, you are giving yourself the greatest number of degrees of freedom of movement, to go forward, backwards or sideways depending on need.

There’s another too, and this is one that was brought home to me as a spectator last night: Tyres. Big, round and, on lorries, often the cheapest possible remould that the operator can (il)legally get away with. These are explosions waiting to happen —  we all take avoiding action on a regular basis to avoid bits of tyre carcass sitting on the road. Well, ever wondered what it’s like to be in the vicinity when this happens? Last night, on the A3 dual-carriageway, I’d moved to the outside lane to overtake a large crane that was travelling at about 50mph in the inside lane. A slow camper van was just clearing the front of the crane and a Audi was waiting behind — right alongside the rear wheels of the crane. I was a couple of hundred metres back, waiting for a clear overtake. Then the world ahead went away, completely obscured by a huge cloud of dust, apparently from nowhere —  there was no sign of the Audi, and the crane had got an impressive wobble on. Emergency stops in the outside lanes of dual carriageways aren’t recommended procedure, but there wasn’t really much choice —  fortunately the motorists behind me seemed to be on the ball and I wasn’t about to be rear-ended. Stop. Wait for dust to clear, then proceed to pick my way through the wreckage: the crane driver was out of his cab, peering in confusion at the tattered shreds of one of his offside rear tyres; the Audi was stopped on the verge in front, with the driver peering in confusion and disbelief at the nose of his car. As I went past, a quick shoulder check showed a bonnet (hood) and front that looked as though the wrath of a very unhappy rhino had descended upon it.
Like, I suspect, most bikes, a Ducati is not well equipped to withstand the ravages of a charging rhino —  it just doesn’t seem to figure in the design specs. Besides, no matter what happens to the bike in such circumstances, it’s not likely to be of much interest to you as the rider —  you’ll probably have already been blown into the next County and/or next life by the blast. Now look at a busy road, in or out of town —  how many motorcyclists do you see either squeezing right alongside the wheels of moving large vehicles or simply sitting there beside them, with nowhere to go in the event of anything going pear-shaped? Right. Think about it, eh?