This is one of the most common grammar mistakes people make.
The natural assumption is that the longer sound has the double-o and the shorter sound only one.
But this is one of the few words where the opposite is true.
If your belt is too loose, you may lose your pants. (Which is fine if it’s “no pants Friday” but any other day of the week, this is a bad thing.)
Here’s an easy way to remember it. >>>>>> Loose has the same amount of o’s as goose.
“Loose like a goose”
Lose has only ONE o. Because it lost one. Get it?