Some people sell their bitcoin because they are tempted and want to enjoy the profit they will get from it in physical bills, some gets panic when its price falls down, while others really just want money to buy something. For me, if you're going to sell your bitcoin leave atleast half of it. Bitcoin's price increases oftentimes so if you still hold some bitcoin, you'll earn profit at the same time. Saving or holding your bitcoin can give you more profit especially in the long-term, no risks involve. In addition, if you noticed continuous increase in bitcoin's price, expect the next dip and that is the right time to buy bitcoin again.