Why do guys act weird/short/have an attitude after hooking up?

Ive noticed many times that when I hook up with a dude, once its done they arent friendly anymore. Like when we first meetup and during sex they are friendly and talkative but then the second its over they are suddenly very short and cant leave fast enough. Not every guy, every time but it’s been a lot of guys a lot of times - to where I’ve noticed a pattern.

Like last night, I hooked up with this dude that was significantly younger than me so there’s not a chance in hell of any type of like romance or relationship or any shit like that . So it’s not like I’m chasing him down for a relationship now or something. Hes young, he’s in the military. He’s probably hooking up with a lot of chicks. But he was nice as could be before we had sex and then as soon as we had sex he was just like “all right I’m out. I have to work early.” And then got dressed and left. But he was totally sweet the whole time up to that point. So I’m just trying to figure out like what is the thought process like what the fuck is going on when this happens?