I suspect it's because he made a correct prediction and wanted to brag about it

Joe Crowley, she ran against him and I said, “You know, Joe, you’re gonna lose this thing unless you go out and campaign.” I watched her campaigning a little bit. I happened to turn on television, I saw her campaign. It was almost like by accident. And I said, “You’re going to have to run a little.” “Oh, well, no, she’s a young girl,” he said. She’s a young girl. She’ll be-, I’ve taken on many of them.” And he ended up losing, pretty easily actually

He probably thinks she doesn't stand a chance OR he thinks it's cute that someone thinks they are going to run for President.

He thinks she would be an easy opponent and thinks he's playing chess by giving her a boost. You know, like Hillary did with Trump.

He's treating her like a child.

Trump thinks he's a mob boss that gets to dictate other people's worth.

I am pretty sure they want AOC vs Erika Kirk.