Guy's Behavior

Why is this boy being mean to me all of a sudden?

For some context, we’re high school seniors I’m 18 and he’s 17 soon to be 18. We work together at a clothing store in the mall and I’ve only known him for less than a month. Let’s just call him Max. In the beginning, he was really nice and playful. Like he’d come over to me all the time, nudge my arm, chase me into the fitting room, try to sit on me, silly stuff like that. When he asked about my plans, I told him I was going to a carnival and he pouted saying that I go everywhere without him. So of course, I invited him to meet me and my friends there. We didn’t end up going because it was raining, so I went the day before with one of my girlfriends. He found out and texted me “Just wowww” and then didn’t open my text since then. Then today, that’s when things started getting weird. He gave me a high five, squeezed my hand, but then started ridiculously picking on me the rest of our shift. He spent the whole shift saying ew when I walked past him, sneaking up behind me, and even when I was ready to leave, he said “Finally.” Like jeez. I also saw one of our co-workers who came into the store (who we’re both friends with) on his off day, and when I told Max that I was happy to see our co-worker, he said “So what? I haven’t seen him either.” And then he mocked my voice. Like wtf. I didn’t let it get to me though, I always give ave him a cheeky grin and the middle finger whenever he says crap like that. I’m just confused because he was pretty nice before, not that he isn’t now, but he’s acting really weird. Especially when he comes over to my side of the store and just stares at me until I look at him.
Why is this boy being mean to me all of a sudden?
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