Sources close to Comey claim that Comey told them a very different story. They were told that the meeting was requested by Trump. During that meeting Trump asked Comey whether he would be loyal to the president. Comey responded by saying that he would be honest. Trump subsequently asked Comey whether he would be "honestly loyal". That was not the answer that Trump was looking for. He demands total loyalty from his foot soldiers like any Mafia boss would do. Comey believes that his downfall with Trump began in that meeting. Trump watched Comey carefully during his interviews in public meetings with congressional committees. He was displeased with Comey's version of honest loyalty. The FBI director was in a powerful position which could be harmful to him if he did not get complete loyalty. Trump has been caught telling lies or providing misleading statements hundreds of time since he was elected. If we have to choose between a story told by Trump or one coming from Comey it would be a "no contest decision".
The real reason my be something like this:
Recounting his decision to dismiss Comey, Trump told NBC News, “In fact, when I decided to just do it, I said to myself, I said, ‘You know, this Russia thing with Trump and Russia is a made up story, it’s an excuse by the Democrats for having lost an election that they should have won.’”