A better PG (Irving) came available at the same time that our best player (IT) was injured and was heading into a very expensive FA period.
As far as we know, Irving wasn't available prior to this summer.
Given new information, why should Danny not try to pursue the best available talent (Kyrie).
While it does suck that IT's feelings were hurt in this process, it is a business. Every player is told that it can be a cold business as a rookie.
I feel for IT, as it just seemed like he was in a stable, good spot for the first time in his career. At the same time, I think that he will find security and happiness in another city. He will be fine.
He's justified for being upset, and I'm justified to be excited b/c the Celtics have a brighter future after the trade.