In a nondiabetic person, the body has more than a dozen responses to low blood sugar.
If you took my (presumably totally nondiabetic) brother, and shot him up with 15 units of insulin, here's some of how his body would respond:
First, his blood sugar would drop. Insulin is insulin.
Once his blood was low, his body would do the following, roughly in this order:
Turn down own insulin production, not quite off. This makes sense, right? If you're low, you don't want more insulin.
Turn up glucagon production. This causes release of glycogen from the liver, raising blood sugar.
Release cortisol- this creates insulin resistance and is also a hormone that helps the body deal with stress.
Release epinephrine aka adrenaline. In addition to possibly alerting him that something was wrong and making him eat, this would cause insulin resistance.
Release norepinephrine aka noradrenaline. Dunno what this does.
Release growth hormone. This is a hormone that is made partly in response to the presence of too much insulin and low blood sugar. It also causes insulin resistance.
Release pancreatic polypeptides. Don't know why, except perhaps because of decreased insulin secretion.
If he goes low enough, then his body will go into an emergency state- it will cut off circulation to his extremities and stomach, and after a while he will have a seizure.
Cutting off circulation to the extremities, in addition to being something you can feel that may alert you to a problem, will help keep the needed sugar by the muscles (heart and lungs) that really need it, as well as by the brain. The seizure will also help cause insulin resistance and raise blood sugar further.
Now, let's take a look at what would happen if I got 15 units of insulin.
First, my blood sugar would fall. I'm not insulin resistant either.
But here things fall apart.
I can't turn off my body's basal secretion of insulin; it doesn't secrete insulin. I can't take the insulin out of my body. Even if I were an insulin pumper, the insulin I'd be able to stop taking wouldn't raise my blood sugar for at least another 15 minutes and probably quite a bit longer.
Because the various hormones the body makes in response to hypoglycemia are a little dangerous (adrenaline can cause heart attacks), it is healthier for it to only respond strongly if it's really needed, so it has a threshhold for when it responds. Because I am frequently low, my body will not respond to a low until I am a lot lower than my brother.
Next, because my type 1 diabetes is not new at all, I will not make any glucagon in response to hypoglycemia.
My body CAN make adrenaline, noradrenaline, growth hormone, and so forth. And it will, a bit. Two things stop it. First, I've been low in the past. My body does not want to react to lows too aggressively, and it seems I've been low before. So it lets this go. Second, the body's ability to do these things are impacted by years of diabetes- autonomic neuropathy.
My body CAN go into emergency mode. As far as I know, this ability is not impaired at all in diabetics. Yay body!
Despite the assumptions of the general public, people with type 1 diabetes are less able to respond to low blood sugar than are nondiabetics. Despite the assumptions of most type 1 folks, we can and do survive many undetected episodes of hypoglycemia during the nights.