Cause of T1D

TL;DR: We don't know what causes the immune system to do that. But we do know what that is.

T1D is an autoimmune disease, meaning your immune system, which is supposed to protect you against things like viruses and diseases and whatever your ex put in your Big Mac, mistakenly attacks something good for you - something it shouldn't be attacking. In this case, the immune system attacks beta cells in the pancreas, which yields very bad results for your health.

Beta cells secrete a hormone known as insulin, which manages blood sugar. Blood sugar can't be too low or too high and it could be deadly in either scenario, but, as dangerous as low blood sugars are, they are rarer because food, stress, and many other things can easily raise blood sugar. Insulin, however, is much more difficult to find than that sugary beverage I know you had too much of last night, especially if your body can't produce it because your immune system went a bit crazy. Thus, people with type 1 diabetes have to find some way of getting that insulin so they don't die - whether that involves injections (terrifying!) or an insulin pump (also terrifying, but at least it doesn't sound as scary).

We're currently not sure what makes the immune system attack beta cells. It's suggested that it's a combination of genetic and environmental factors, and it could be caused by viruses or infections, which is a fancy scientific way of saying we don't know anything about the root cause of T1D. Until we do find out, however, and hopefully develop a cure as well, people with T1D will have to continue using insulin.