Why would a guy act tough in front of a girl?

There is no point, unless some other guy is mistreating you or trying to slap you around. He may just be over protective. Not a matter of being macho, maybe the guy means well but has a short fuse for stuff like that. You’d have to give is more input on the nature of the situation.

To scare off the competition?
If so be careful of possessive people.

To look like he can protect her/rescue her?

Topically. Insecurities.

“Macho” behaviors can be seen as mating behavior. The strongest males are instinctively perceived as fittest for procreation. Even though a lot of (most?) women can’t stand macho behavior, there are enough who like it that they breed with the biggest jerk they can find because… instincts, and we get whole families of “though” jerks down through the ages.

Yeah. To a certain extent all guys get to posturing in some way in front of the girls. It’s what organized sports are all about, really.
I watched a program about LeBron James. He said one of the most important things about playing is it got the girls!

I won’t speak for all males. But. Wherever I have been, you have to be tough.
Going back to grade school. If some guy bumps shoulders with you. It’s a sign of disrespect. And. Needs to be dealt with.
Otherwise. It’s a show of weakness.
You don’t get a lot of chances.
Other males, will challenge you.

That’s not necessarily, around females. It’s just, how things are.

Back to the thread.
Some guys, feel that it is more important to display their masculinity, in a female’s company.
So. They go overboard.

The behavior is likely instinctive, or primitive. However.

It can easily become an issue.

In my experience. It’s common to see a male, get in over his head. Trying to prove himself.

I suppose it is just a matter of, essentially, worthless and outdated thinking.
It’s definitely more common, with smaller males. One could call it, an attempt at over compensation.

As a former bouncer, I’ve seen it before. It usually doesn’t end well, for the “tough guy.”

They end up in an ambulance.

Some guys, just don’t learn anything from a good ass kicking…

I appreciated a guy who could protect me.