This is the super simplified version because they were explaining it for about 20 minutes.

I also think it would have made Doffy a bit softer having someone to deeply and obsessively care for, something stronger than his hate.

I like to think Rosinante’s sense of self was unstable, especially with his trauma, and that he was easily influenced by the people around him and his environment. So growing up with Doflamingo he turned out a lot different than as a marine.