Why do i need to start and end my anti-depressant gradually. what will happen if i don't?

Antidepressants: With most antidepressants, dose is gradually titrated upwards to therapeutic levels. This is so that your body can become accustomed to the medicine, and any side effects (if they occur) will be less overwhelming. It's wise to taper antidepressants so that you don't experience discontinuation symptoms, and so that you & your doctor can respond to any potential return of depressive symptoms.