Well, Cato the Younger was a reactionary politician of the Late Republic. He was staunchly against the Populares, the reformers in favor of the poor. Caesar was one of the most prominent and consistent Populares of the period, and Cato decided that, for that offense, he was gonna hate the fuck out of Caesar.

Land reform comes up? Filibuster. Caesar running in an election? Bribery to (unsuccessfully) try and stop him. The Senate and Caesar coming to an agreement to prevent a civil war and preserve the Republic? Scuttled because Cato wanted Caesar dead. Demanding Caesar surrender to the Germans, abusing the power of the court to prosecute political enemies, fake religious omens every day to prevent any government business from being done. He claimed to be the defender of Republican traditions while violating Republican traditions at every turn.

For obvious reasons, him and Caesar didn’t get along.