Blockchain Network
Each has a specific way that fees are paid out. For example, Bitcoin is associated with higher transaction costs than Ethereum.
Transaction Size
The higher the transaction (in bytes) equals a higher fee. However, sending large amounts of cryptocurrency might call for bigger transaction sizes, which often result in higher fees.
Network Congestion
Transaction fees tend to go up and congestion of the network increases when it is on a very high demand. This is very prevalent among common networks such as Bitcoin.
Urgency
You may want to pay a high fee in order to have the service completed quickly.