There is much debate on this topic, both by modern scholars as well as ancient scholars. Suetonius claims that Caesar believed it to be his destiny. He tells a story of Caesar seeing a vision of a supernatural being taking the trumpets from one of the Cornicens in Caesar's legion and crossing the rubicon, encouraging Caesar to follow. Suetonius also tells us, however, that Caesar also needed a civil war for his own personal political agenda. Many modern historians believe that it was out of desperation. Caesar had, in the months prior, been trying frantically to settle with the people in Rome so that he could run for the consulsip, but it was essentially a sure thing that he would be put on trial if he set foot in Rome again. Suetonius that Caesar justified his actions by claiming that he needed to defend the tribunes, who were threatened when they supported the Roman Imperator (which here means military commander, not emperor). Caesar really had nothing left to do. He couldn't return to Rome to defend himself because he would no longer have the protection his imperium (magesterial power) and would be put on trial, and a series of miscommunications had basically ruined any chances of avoiding civil war. For a more in-depth explanation, see Goldsworthy, Caesar: Life of a Colossus (Yale, 2006). Also see Goldsworthy, Caesar's Civil War, Osprey series. For primary sources, check out Suetonius, "Divus Julius" and Plutarch "Julius Caesar". hope this helps!