He Chose to be in Miami over anything else
While free-agency explicitly implies that the “agents” or players are “free” to choose wherever it is they want to go, this is especially the case in the Jimmy Butler scenario. Perhaps it was his relationship with Dwyane Wade, all of the things he mentions that he has heard about the Miami Heat Culture from those like Wade, or the influence that a small trek through some of the cities popular districts had on him(such as the appearance in Domino Park while still a member of the 76ers), that led him to this decision.
But regardless, he chose to be where he is at the moment. While there are surely other factors to get to here, this is one of the most important at play.
While some decisions or choices that we make as individuals turn out to be the wrong ones or the ones that aren’t eventually the best ones, they are treated as learning situations more than regretful ones on most occasions. This is simply due to the fact that although it may have not worked out the way that the person wanted it to or planned for it to, it was still an individual and personal decision on behalf of the person themselves.
That is the first reason why he won’t regret it, because at the end of the day, he made it on is own and for his own personal benefit or not.