A deposit creates a liability for the bank. It also creates an asset that can be used to generate a return. It also creates various operational costs. The way

A deposit creates a liability for the bank. It also creates an asset that can be used to generate a return. It also creates various operational costs. The way those three things play out the more likely it is for a depositor to suddenly ask for a lot or all his money back the more risky it is for the bank. Both in terms of creating a bank run and just in terms of losing money on the relationship.