They have lived together for more than 24 years, yet at times it seems they have never met.

Marcus Smart’s head and heart are prone to quick turns in separate directions, and wisdom is often the loser.

On Saturday night in Cleveland, Aron Baynes and J.R. Smith got tangled in the lane as they awaited a rebound that never came. After Terry Rozier hit his jumper, Baynes tried to free himself to get back on defense, spinning the smaller Smith down in the process. Smith got up and shoved Baynes.

Smart rushed in and did likewise to Smith, then got into a fighter’s stance and kept up the challenge.

On Monday, the NBA shoved back, giving Smith a $15,000 fine for initiating the altercation and Smart a $25,000 hit for escalating it. Fortunately for the Celtics and Smart, there was no suspension.

The league tends to come down harder on those who enter an altercation as opposed to those who are in it, which is why Smart was ejected Saturday while Smith was not.

It’s just never really a good idea to come flying into a fray, particularly from behind. The best/worst example of this came in December 1977 when Houston’s Rudy Tomjanovich came quickly to the aid of a teammate only to have Kermit Washington of the Lakers turn and cave in his face with one punch.

Smart was right to want to support his teammate, but wrong to do it in a manner that cost him and the Celtics – and could have led to much worse. So while maybe his heart was in the right place, he didn’t go about things in a helpful way.

It’s appreciated by his team, teammates and fans that he plays with that type of passion. It’s a large part of why a guy who averaged 10.2 points and shot 36.7 percent last year got a four-year, $52 million contract.

But in each moment – in each snapshot – there’s a bigger picture. And Marcus Smart has to consider it.

That is not, however, in his nature. It wasn’t when he shoved a fan at Texas Tech in 2014, and it wasn’t when he took out his aggressions on a picture frame in his Beverly Hills hotel room last January.

Life would be so much simpler if we could all act on our impulses without negative consequences.

On Saturday, it took two Celtics to tackle Smart and keep him away from greater trouble. He still managed to give Smith the finger and invite him to continue the matter in the back hallway.

Later, even after a couple of hours to cool down, Smart was extremely emotional when he said, “All that on the court, we can handle that off the court. I ain’t with that. And that’s on my mama, may she rest in peace. Ain’t no punk right here.”

Smart pointed to a tattoo of his recently deceased mother and said, “On my mama, may she rest in peace … J.R. knows where I’m at. Everyone knows where I’m at.”

There is no way to tell how Camellia Smart’s loss to cancer on Sept. 16 played into Marcus’ reactions. As I’ve said before, among the things I learned from my own mother’s unexpected passing is that no one really knows what anyone else is going through.

A large part of the reason why Smart’s behavior in these situations is confusing in any way is because he is one of the nicer and more thoughtful people I’ve ever dealt with on this beat. At his summer youth basketball camps, he is much more than a drive-by name on the banner. He interacts with the kids and appreciates them.

In other settings, however, he can get quite different in a second.

But let’s examine Saturday’s situation. First of all, Smart should have recognized that Baynes was more than capable of dealing with Smith on his own if that was his choice. (A flick of Aaron’s pinkie might have done it.)

Second, if he wanted to get back at Smith, the wiser way would have been to bide his time and set a hard pick later. Maybe even deliver a purposeful foul. Just something in the flow of the game that would have kept Smart on the floor.

Asked later that night if he wanted his players sticking up for a teammate, Brad Stevens said, “Yeah, I mean, we want him in the game.”

And for the last 39 minutes and 38 seconds that night – albeit the last 39:38 of a meaningless preseason game – Smart was not in the game.