We should explain ...

31-year-old Gaethje says the way he's built, he goes "balls to the wall" in everything he does -- and that little voice that most people have in their heads telling them to "slow down" and "be careful" ... yeah, he doesn't have that.

He doesn't WANT to die -- but once he starts to do something that might be dangerous (like riding a motorcycle), he'll push it to the limit with no regard for his own safety.

Hence, his rule of trying not to put himself in a position where death is a real possibility. Get it?

Here's how Justin put it to us while talking about his upcoming fight with Khabib Nurmagomedov at UFC 254.

"I'm confident in my skills, not one bit scared to step in there. I'm looking forward to that feeling. I chase that feeling. That's why I don't buy motorcycles because I will die. That's why I do a lot of things to keep myself from even putting myself in positions where I can be in danger because once it starts, it's balls to the wall."

Scary as hell, right?!

There's a lot more ... as far as Justin's MMA career, Gaethje says he's got "at least 5 more fights" -- but he'll only count a fight if it's a "war."

"If they're not wars, they don't count."

The money is also a factor -- "As long as [the UFC is] paying me as much as they're paying me, I'm probably gonna fight until I do not believe I can be the best in the world anymore."

Gaethje says the money he'll make from the Khabib fight will mean his mother no longer has to work another day in her life.

The only issue ... mom -- a Postmaster at Safford Post Office in Arizona -- isn't ready to retire just yet!

"She's so crazy. She's like 'Son, I can't leave them for Christmas. I have to at least work until Christmas!' I'm like, 'No you don't! You don't owe them nothing!'"

"But it's just a testament to who they are and who I am."