Your body isn't meant to handle a lot of sugar at one, especially not in liquid form. Eating fruit is relatively ok. Blending it into a smoothie and then drinking it can be bad. By blending, all the fibers get broken up so it becomes extremely easy for your body to digest. This then causes a massive blood sugar spike. Repeat this on a regular basis and you will get diabetes.
(The fiber of non-blended fruit is harder to digest casuing the sugar to be gradually released over a longer period of time)