How Do You Become Diabetic?

Your pancreas stops working as it should.  It slows down or stops producing insulin.  This in turn causes a high sugar count.

2 ways:
1.  When your pancreas stops producing insulin (for Insulin Dependent Diabetes Mellitus - Type 1)
2.  When your cells are resistant to insulin (for Non-Insulin Dependent Diabetes Mellitus - Type 2)

Usually insulin resistant cells are those cells that form the adipose tissue or fat cells.  That's why they are mostly adult onset, and happens to those who are obese individuals.