Those are good points made in the posts above. There was anger, motivation, but also definitely gamesmanship.
He would often use tantrums to disrupt the flow of a match, try to change momentum, scare linespeople, and get some more time between points.
(it's absolutely UNACCEPTABLE..etc)
(outburst vs. Lendl in 1988 French Open)
Often, when he started losing a match, he would start delaying, and angry outbursts would follow, which would tend to change the momentum somewhat, especially if he was getting tired and needed more time between points against an opponent that could beat him that day.
His matches against Connors could be extremely explosive (they hated each other back then). You would actually fear that the match would end in a streetfight between the two. Check out these links with him and Connors. Basically, Connors would not "take it", but actually it was also
because Connors was not used to someone using the same tactics (gamesmanship) that HE was used to employing at times:
(they get real close to exchanging blows)
(French Open meeting at the net)
(just good tennis)
(relationship getting better)
(actually practicing together by 1991)
Now, he would NEVER act like this against Borg, because he would look really bad. Borg and he actually got along real well, but there was a lot of mutual respect. McEnroe said also "he could waste no energy" against Borg.
In fact, McEnroe would rarely even say a word when playing against Borg. Once, Borg actually complained about a line call, which was a famous incident. Yet, McEnroe actually displayed very good sportsmanship, by intentionally giving away a point to try and "erase" a previous bad call that went against Borg (indoors at the Masters, three set match). I don't quite understand this behavior against Borg, but I found out that when he came on the scene at 19, Connors was very "condescending" so to speak, but Borg "took him under his wing" and accepted him, which helped him out.
The personalities of McEnroe, Borg, and Connors were another aspect that made those rivalries very intriguing.