Kinda sad in one way. Sessions was an early Trump supporter and I think he's a good guy. He's clearly on the side of law and order and was getting some good stuff done. But he clearly was not the right temperament for the job and obviously trusted his fellow Justice Dep't people far more than they deserved.Read more here.
His failure to act over and over when it was clear as a leader he should (such as when they were stonewalling Congress on documents, defying direct orders from the president, etc) made it obvious the man was not fit for the position at this time.
If we had a good DOJ that had not spent the last 24 years being infected by hard left radicals, he probably would have been the perfect choice. But he either refused or could not see how many people were in his department who were far more interested in leftist power than justice and law.
And so he needed to go.
To me it is a signal that Trump is not going to put up with any more shenanigans from the DOJ people like Rosenstein.