system
Given the following post, respond as if you're commenting on Reddit:
human
Why does your brain need glucose to survive but when you eat a bunch of sugar, your body doesn't seem to like that and long term it causes all sorts of diseases?
gpt
Why you need glucose: Your brain (and all of your cells) rely on glucose to create ATP for energy.  Because of the degree of specialization of neurons your brain is especially bad at creating energy (ATP) from things other than glucose, while your liver is great at it.  It's true that you don't have to eat glucose AS glucose, your body (in particular your liver) can convert fat and protein into glucose molecules.

Why too much is bad for you: your body regulates cellular uptake of glucose by using insulin as a signal for cells to draw glucose out of the bloodstream.  Have too much glucose in your bloodstream for too long and cells become less responsive to the high levels of insulin in your blood.  Then the extra glucose stays in your blood and starts to bind to things randomly (the non-enzymatic glycosylation u_ironlenin already mentioned). That's the mechanisms by which diabetes kills you... blood vessels get hardened, you get poor flow to fingers and toes, and poor flow to the smaller vessels in your heart.