Men, why do you act different around the girl you’re interested in? Wouldn’t it be better to act the same?

There’s a guy at my gym who I finally got the courage to approach last week. Things went okay, I guess. I noticed that with me he seemed to put on a more serious face and not smile or laugh. However, today at the gym this random girl came up to him while he was in a conversation with another guy and started flirting/hitting on him from what I could see. I noticed that he was immediately laughing and smiling with her. What’s that all about? Or am I wrong in thinking that he’s interested in me? Just for some background this is a guy who asked my friend on whether or not he should approach me a few weeks ago but I was too nervous to go through with it. So who knows maybe things have changed. I just wonder why he’d be so serious with me and not laugh or smile but with this random girl it was instant smiling and laughing. Like even when he tried to tell a joke to me I guess my face looked so confused about it that he didn’t even try to laugh or smile after saying it. Idk, just wondered if this is a way a guy would act if they were interested in you.