Why is the relationship between Brutus and Cassius strained?

Brutus has brought his armies to Sardis (now Western Turkey) and has set up camp. A messenger whom he sent to Cassius informs him that Cassius is not as friendly anymore. Brutus remarks, "Thou hast described / A hot friend cooling" (4.2.18-19). At that moment Cassius' army arrives and Cassius himself appears. He is angry with Brutus and starts to accuse him of wronging him, but Brutus makes him enter the tent so that they do not appear to be fighting in front of their men.

Cassius is upset that Brutus publicly disgraced a friend of his for taking bribes from the Sardians. Apparently Cassius had sent several letter to Brutus urging for the man's release, but Brutus refused. Brutus is furious that Cassius would even consider defending a man for taking bribes, arguing that Caesar was killed for exactly such behavior. He states, "What, shall one of us, / That struck the foremost man of all this world / But for supporting robbers, shall we now / Contaminate our fingers with base bribes" (4.2.73-76). Cassius and Brutus end up threatening each other, with each man convinced he is better able to lead the armies than the other.

The two men continue arguing, and Brutus finally tells Cassius that he is upset that Cassius refused to send him gold with which to pay his soldiers. Brutus says, "I did send / To you for gold to pay my legions, / Which you denied me" (4.2.130-132). Cassius denies it, and in exasperation pulls out his dagger and offers it to Brutus. He tells Brutus to kill him if he is such an terrible man, but Brutus recants and they finally embrace in friendship.