Why is Pip finally able to accept and love Magwitch? What has caused this change in Pip?

As a result of the concern of Herbert and Pip for Magwitch's safety and Pip's growing affection, Magwitch becomes, in Pip's terms, softer or humanized. In court when he is referred to as a desperate criminal, he looks at Pip

with a trustful look, as if he were confident that I had seen some small redeeming touch in him, even so long ago as when I was a little child. As to the rest, he was humble and contrite, and I never knew him to complain (page 461).

As he lies dying in the prison infirmary, Magwitch appreciates the fact that Pip has been closer to him and more accepting of him in his fall than in his prosperity. I think Pip is through with judging people so harshly. Pip is a lot more sensitive and mature. He sees motivations in people in complex ways.