I believe the concept of HODLing Bitcoin is nearly as old as Bitcoin itself with the fundamental reason and hope that it will increase more in value over the years. Although several persons are entitled to there own reasons for HODLing, could it be that some persons HODL because they are waiting for the perfect time or they are too scared to sell or lose a percentage of their Bitcoin?
An investment is a strategic plan and should be treated as such, the length of time a person should hold should also be planned, the decision to sell should never be done on impulse, you don't hold bitcoin because you want the price to jump for you to sell, you hold bitcoin so that your cumulative holdings will appreciate in value with time, this is more or less the point of every investment done out there, so for bitcoin, no one should HODL out of fear, investors should HODL because it's part of their investment plan.