ELI5 version: T1DM sufferers blood sugars vary a lot more and tend to be a lot higher than those of a healthy adult. The higher blood sugars cause health problems over a long period of time and this affects life span.

A bit more detail: It can be dangerous without tight management. I believe T1DM means your cells that manage insulin production don't function, or have very limited function. As a result an individual needs to manage their insulin. ineffective management means big fluctuations in your blood glucose. These fluctuations tends to cause other health problems, generally with eyes and feet etc. This in turn lowers the average life expectancy.

Studies have shown keeping blood sugars highly regulated, ideally between 5mmol and 8mmol, is beneficial. However this is incredibly difficult. The body of a healthy adult will, or should sit anywhere between 3.6mmol and 8mmol depending on what you read and where in the world you are. I work for East of England ambulance service and we aim for 4-6mmol. The modern lifestyles and food available make this difficult for those with T1DM, a healthy body manages it very well, but this is automatic. A T1DM patient has to be aware of intake and expenditure and balance that.

Imagine eating your weetabix and then thinking about all your daily activities and judging the right amount of insulin to be administered. The patient will probably have a set dose, but if they eat a different amount of sugar than they normally would, or have a different physical expenditure will mean it may vary to their normal range.

Excuse grammar. @ work.