A Short answer, to save his life. Long answer, he was an ambitious man who would rather be the first man of a barbarian village than the second man of Rome.

Plutarch relates this story here;

We are told that, as he was crossing the Alps and passing by a barbarian village which had very few inhabitants and was a sorry sight, his companions asked with mirth and laughter, "Can it be that here too there are ambitious strifes for office, struggles for primacy, and mutual jealousies of powerful men?" 4 Whereupon Caesar said to them in all seriousness, "I would rather be first here than second at Rome." 5 In like manner we are told again that, in Spain, when he was at leisure and was reading from the history of Alexander, he was lost in thought for a long time, and then burst into tears. 6 His friends were astonished, and asked the reason for his tears. "Do you not think," said he, "it is matter for sorrow that while Alexander, at my age, was already king of so many peoples, I have as yet achieved no brilliant success?"24

so we know that Caesar was an ambitious man. When he vied for the office of pontifex Maximus he went so deeply into debt that he told his mother he would end end up with the office or in in exile. Again we have this from Plutarch ; mother thou son shalt be pontifex Maximus or in exile

But you can see from his own writings that he thought very highly of himself. Of course these works were designed specifically to swing public opinion to his favor. He was writing to an audience that he particularly wanted to win over.

Now then, as to specifics, he particularly wanted to stay alive. It was an interesting custom among high roman officials that they were immune to prosecution while they were in office. This gave them leave to do what they felt necessary to maintain the peace for the empire, but there was a check and balance to this system.

Any high official was immune to prosecution while they were in office but as soon as their office expired, they were subject to prosecution like anyone else. If they had high powered officials as enemies, as Caesar did, this likely meant the death penalty.

Caesar had many high powered enemies. He had managed to arrange a deal between the most powerful men in Rome and himself. Crassus, the richest man in the empire, and Pompey, the greatest general Rome had seen to this point. Through this triumvirate, he had managed to get himself appointed as governor of Gaul for five years.

While in Gaul he had been able to subjugate countless people and conquer huge amounts of land for the empire. But given his enemies in the senate, he was after all a populari, he was ultimately declared an enemy of the state. Mathis meant that as soon as his term of office ended he would likely be prosecuted and executed for, what some deemed, his illegal with the Gauls.

Caesar was not the kind of man to walk quietly into that long dark night. When his second in command, and tribute of the plebes, Marc Antony was assaulted by the senate, and himself declared an enemy of the state, he took the opportunity to show his men, who had grown very prosperous under his leadership, that the senate was corrupt and Rome needed to be reminded of who it was that made them the great empire they had become. So he led the thirteenth legion to Rome.

At this point Pompey the great was consul and had assured the senate that he could raise legions to defend Rome. The problem was, all of his actual legions were in Spain. They didn't do much good against and advancing, very popular general, who had actual battle hardened men at his back. So Pompey and the senators that opposed Caesar fled leaving Rome wide open. Caesar marched in unopposed.

The senate remained were friendly to him and, ultimately, declared him dictator to deal with this civil war.

It isn't really clear whether he set out to take the empire into his own hands, but it seems that he was largely responding to to the situation that presented itself to him. He wasn't going to simply let the senate kill him, and he was a supremely ambitious man. He was, in the end, a better general than anyone else in the empire, including the amazing Pompey Maximus. And if he had not done what he did, he would have been killed by the senate.

Of course history is never this black and white. There were points in negotiations with the senate where all of this could have been avoided, but both sides were too stubborn to see their way to that.

I guess the tl;dr is that Caesar was too ambitious to be anything but Caesar, and the senate too stubborn to to anything but fight him.

There is way more too this story than the little I have conveyed here, I will try to answer any questions you have, but hopefully those with more information can do a better job than I can.