Daniel Theis shows his frustration during a loss to Miami in Game 1 of the Eastern Conference finals on Tuesday. Mark J. Terrill/Associated Press

The NBA conference finals are supposed to be hard. Miami is very good, and will find ways to score against even the best defenses.

So it’s in the best interest of the Boston Celtics to not be their own worst enemy.

“We got stagnant,” Marcus Smart said after the Game 1 loss. “We rested and we got complacent, and against a team like this, you can’t get complacent.”

Complacency has been an issue for the Celtics for a long time.

It was an issue back in January when, after three straight losses, Smart said “we are trying to figure that out. We just haven’t played hard. They got easy transition baskets on us because of our mistakes.”

It was an issue a month ago when, after a seeding-game loss to these Miami Heat, Jayson Tatum said “we just have to stay focused. Basketball is an emotional game and we let our emotions play too much of a part today. We just got to do a better job of just focusing on what we can control and not worry about other things.”

It was an issue 10 days ago when, after a rough Game 4 loss to the Toronto Raptors, Tatum said, “They played harder than we did and I think that was just noticeable on both ends of the floor for most part of the game.”

This Celtics team can be fun and exciting at its best. When it’s all clicking, players can fly around both ends of the court and treat fans to some honestly dazzling basketball.

When they’re not? That’s when 14-point fourth quarter leads vanishes in a haze of fouls and transition baskets.

It has been difficult for teams to operate under these circumstances – 70-plus days in the bubble environment tests one’s mettle. Outwardly, they are all blessed individuals getting paid quite well to play a game., and 9-to-5ers don’t want to hear about how hard it is to live at Disney World for a few months to play basketball.

So they are not allowed to express their outward frustration of being trapped in an isolated environment.

“I’ve said this a thousand a times but that’s why, if you’re in this, you don’t scrutinize with the same level of eye than if you’re not just because you know how hard it is,” Stevens said last week. “You know the physical toughness it takes, the emotional toughness it takes, the mental toughness it takes, and I believe that we have that.”

Stevens might be proven right about their toughness, but the Celtics are going to have to do that by addressing the one thing that has plagued them all season.

Tatum was asked on Monday about the lessons learned from the semifinal series against Toronto. “To finish out games,” he said. “Obviously Game 3 kind of changed the whole series. We was half a second from going up 3-0 to playing in a Game 7, so finishing out each possession, each game, and just how critical that is because you never know how it can change the outcome, the dynamic of an entire series.”

Tatum was as guilty as anyone of stagnating Boston’s offense by hunting out isolations against mismatches rather than moving the ball and running a fluid offense. Isolation is easy because it doesn’t require reads or cuts or moving without the ball. It’s just standing, dribbling, and trying to find a sliver of opportunity to get a shot off.

Walker was also asked about lessons on Monday and he echoed Tatum by saying, “I think that’s the greatest lesson, is the intensity of the playoffs and how hard it is to win a single game. Yeah, especially learning how to finish quarters out and finish games out. I think those are a few lessons that we’ve learned.”

The Celtics are young, somewhat brash, and full of extraordinary talent. They’re also prone to failure due to their own mental lapses.

They know what they need to do. It’s there, within them, somewhere. They just have to find it.

“It’s definitely something that can be fixed,” Smart said. “It’s part of the game. You’re not going to be perfect every night, you’re going to make mistakes. You just try to learn from your mistakes on both ends.”

They’d better learn soon.