Why is high blood sugar bad?

Complications. In a diabetic a high blood sugar on a short time basis is a signal to change your treatment regimen. If it remains high on a steadier basis it will promote the development of the complications in the eyes, kidneys, circulation, heart, etc. High blood sugar by itself is detrimental to many of the body's organs and makes the diabetes more difficult to treat.
Diabetes. The sugar molecule is large and Insulin should normally control the levels in your body, with diabetes, the circulating sugar levels can be high, and over time, this can start to cause damage as this large molecule pushes through smaller pores/openings, etc. Damage can occur over time in the kidney, eyes, circulatory/vascular system (foot ulcers, heart issues, erection problems), etc.