Here's the reason people think those players will leave their teams (not necessarily to the Celtics):
Jordan: The Clippers may not want to pay him the max when they have CP3 and Griffin at the max already. It's possible. I don't think Jordan wants to leave, but the Clippers may be the ones who want to let him go. Rivers loves Jordan so I doubt it, but the taxes on the new CBA are no joke.
Love: There have been consistent reports that Love does not enjoy being the third wheel or "Bosh" on the Cavs. There's also that tweet from Lebron earlier in the season and so on and so forth. Ironically enough, I feel like Love's injury may have actually bonded him more with his teammates than ever, but say the Cavs win a championship without Love this season. Do they even want to keep him? Would Love want to come back as the definite third man when he may feel almost 'unnecessary' if the Cavs win this year? That may be a huge ego hit for a young star who is still trying to cement his legacy in this league.
Even Woj (the most reliable 'rumor' reporter in the NBA) has come out and said Love will probably look around the league this summer and that the Cavs are scared out of their boots that he will leave. Then there was the article from Woj saying that the Celtics have closed in on the Lakers as a potential destination for Love if he leaves Cleveland (KO incident may have changed his mind, but who knows). No one is being a 'cockeyed optimist' here. They are just reading reports and coming to their own conclusions. I think there is a better chance you are just being overly pessimistic because there is plenty of smoke here.
I think a lot of the sell to Love will be based on giving him the reigns and ability to play the way he did in Minnesota but in a winning situation. I think luring him will be dependent on if we can make a big trade or not (like for a Cousins, who I honestly don't think will be available until next summer if the Kings fail to win yet again).
Longshot, but: being a 40 win team (same as Minny last season with Love) may be a decent sell in itself in a "see what we can do without a scoring star? Imagine us with you, etc, etc. Remember all that go-to scoring and defensive rebounding that we missed in the Cavs series? You could have helped us turn that series around," kind of way.
Gasol: I agree this is really unrealistic. He grew up in Memphis practically. Only other place that has a shot at him is SAS.
I also think there is almost little to no shot Middleton, Leonard, or Butler leave their teams because like you said, they are restricted free agents. Only shot Leonard leaves is if SAS somehow convinces LMA and Gasol to join their team. SAS may choose to sign and trade Leonard (for a first round pick or two) if they prefer that "twin tower" core over a wing star. I doubt it, though. Only chance Butler leaves is if Chicago implodes in the off-season, and with all the reported drama between Thibs and the ownership anything is possible there, I suppose.
Not sure why Harris is on your list though. He is very gettable. His defense is complete junk, and I doubt Orlando would hesitate to move him in a sign and trade if another team throws a max offer sheet at him. I don't really want him anyway to be honest.