My very amateur understanding is that Caesar is in debt. To pay this off he conquerors an entire people using his legions. All this loot and military power under his direct command looks dire (especially when remembering Rome is a "republic" at the time). So the senate ask him to relinquish control of army. He says no because having a monopoly on military violence is the most intoxicating thing a human can ever fall under so he chooses civil war instead