Why would a guy be protective over a girl?

I would more detail to answer this.

It could be as simple as being raised that way. It makes sense in violent confrontations - who is more likely to be able to stop the assault of a woman by a 200 lb guy - another 200 lb guy or a 130 lb woman?

If by protecting her, he means keeping her locked in a castle tower, that's control.

It depends.

BECAUSE SHE'S MY PROPERTY!!!!!!!!!!!

She has the pizza voucher

Traditionally, it used to be somewhat accepted that men were protective of their partners and kids. These days, protectiveness might be more of a sign to other men to stay away from their territory.

There could be several reasons. Does he feel that she is facing something that might harm her?

I believe there is something primal, almost instinctual, for (at least some) men to protect the one who is important to them if they perceive she may be in danger. Like most instinctive reactions, of course, he could be completely wrong and "protection" (from his POV) could be seen as "interfering" or "controlling" from hers.

Talking about it might be helpful as perceptions and motivations can be unclear. Plus it's a bit of a confusing subject to start with.

As someone once told me: "I don't need anyone's protection. But it is really nice to feel protected."

why do people ask such lame questions on this site?

It might reflect personal feelings for or about the particular girl in question, or it might only reflect a general commitment to traditional gender expectations, but it can easily be both.
Example:
My boyfriend goes barefoot 👣-- like always and everywhere -- but he always wants me to be wearing shoes. 🥿🥿
This might be mostly a romantic contrast thing, but I also see it as protective, and I really like it. 💕

I think we often confuse controlling behavior for protectiveness.

A guy might not , but a gentleman would .

Some of us were raised that way. When I got the car keys from my father to take my girlfriend (future wife) on our first non-double date, he told me to remember that when she was with me, she was the most important person in the world. Obviously, those words have stuck with me for almost 50 years.

Control.

He might like her

Same reason the men of The Titanic allowed the women and children to get into the lifeboats first, knowing many of the men would go down with the ship. It’s in a man’s nature. Well, most real men’s nature.

He don't want anybody thinkin he's gay.

He is possessive

Reason should be because he cares about her. Not an arrogant ego kick.

Could be that he's the jealous type. Could be that he just cares about her well-being.

He wants to marry her.

He's controlling

DNA?

It's genetic

Probably wants to get in her pants ffs 🤦‍♀️

To protect from all the alleged sexual pre dators and aliens out there.

It's hard to know. He could just be a good guy. He could have feelings or he might not. Good people have a tendency to act whether there are personal feelings or not.

usually because he is a gorilla with too much adrenaline.

Vageen

stupidity

Protective and possessive are two very different things.