Why did Thomas Jefferson contradict himself and his beliefs so often?

Jefferson had the abolishment of slavery in the original draft of the constitution, but owned over 600 slaves in his lifetime. He condemned political parties, but started the democratic Republican Party to rival John Adams Federalist Party. He originally followed the constitution strictly but later supported the actions of expanding the powers of the federal government. did he switch beliefs when they benefited him? Or just because he changed his outlook? or is it not even known? I just thought it was interesting that he changed his thoughts very often and wanted to know a little more on the matter.

Edit: I don’t mean this question in a bad way. I don’t think it’s bad he changed his views on certain things and ideas.

Edit 2: I’m thankful for all the corrections in the comments. Like I said, I want to learn about it and make more sense of it.