WASHINGTON -- President Donald Trump explained why some decisions are so tough, in an interview from the Oval Office that aired Monday on "CBS This Morning."

The president said the "bigness of the office" and the "bigness of the deals" made him realize the magnitude of the room. But Mr. Trump, who frequently raises his prowess in making those deals, opened up about why some decisions as commander-in-chief — such as striking a Syrian airbase with 59 Tomahawk missiles in response to a deadly chemical weapons attack — are so tough, and differ from a decision like "going to buy a building."

"Because it's human lives," the president said.

"You're killing people," Mr. Trump continued. "And you can kill the wrong people, too. You know, those things go off and they end up in a town or they end up in a city. And you have another tragedy on your hands. So, these decisions are unbelievable -- you know, in terms of the importance because it's human-- it's--it's--it's killing. I hate it. But things have to be done."

"There's nobody you can call," he added, asked whom he calls to help make those heavy decisions.

The president said his administration has "had some difficulties" with former President Barack Obama, addressing his previous claims that Obama's White House conducted surveillance on Trump Tower.

"I don't stand by anything," referring to his past surveillance claims. "I just -- you can take it the way you want. I think our side's been proven very strongly. And everybody's talking about it. And frankly it should be discussed. I think that is a very big surveillance of our citizens. I think it's a very big topic. And it's a topic that should be number one. And we should find out what the hell is going on."

But the president took a lighter tone in some portions of the interview as well. Referencing a red button on his desk, Mr. Trump said it "gets you a Coke or gets you a Pepsi. One or the other, it'd have to be. Any other cola companies I should mention? Right? But it gets you something."

Mr. Trump also shared how "big people from big companies" have reacted to entering the Oval Office's historic grandeur, including one person whom he said had been to the White House 51 times, but never to the Oval.

"The person came into the Oval Office and started to cry," he said. "This is a tough person by the way. Came into the Oval Office and started to cry. Now, this-- a person with a magnificent office with beautiful glass walls and everything else. You understand. You've seen those offices before. But there is something very special about this space."