Why did he get so distant?

There’s this guy that I really like. He and I are both student workers at a university restaurant. Even though we’re coworkers, I could kind of tell that he was into me. He just really went out of his way to help, even when I didn’t ask for help. He’s even flirted with me a couple of times. He told me once that he was going one weekend to bother his friend, saying “I’m not going to say no to bothering someone at work. I already bother you enough. Or maybe I should bother you more?” He would make flirty comments like that.

So two weeks after he flirted with me, I invited him to my birthday party. He said that he wasn’t sure if he could come, but that he’d let me know during the week if he could make it. He wasn’t able to. But he was acting really weird that week. I saw him two days during the week. The first day, he was kind of cold at times, and it seemed as if he didn’t want to be around me. But the next time I saw him, he was kinder. He asked me how I was, and it was like things were normal. That was the day he told me that he wasn’t able to come to my party. Two days later, my manager announced that he got promoted to department lead, which is basically like assistant manager.

The next couple of weeks, he would fluctuate between being nice to me and being distant. I kind of gave up on him, because he got weird after I made a move. But after I came back from Christmas break, it’s like things were back to normal; before the birthday party took place. He was super excited when the first day we worked together. He gave me a hi five and asked how I was doing. He didn’t get excited like that when other coworkers came back. One time, he came in later, and asked me if it had been busy that day. I said it was slow, and he said “so you’ve just been hanging out here looking pretty?” Another time he counted my register, and the money was correct, so he said “you’re perfect”

I know that obviously the birthday party and his behavior after is and should be a hint. That’s why I gave up on him. Because I figured that he’s not into me and I just really badly misinterpreted his signals. But at the same time, why did he get so excited to see me, and not any other of our coworkers? Why would he flirt with me? If he genuinely didn’t like me, and wanted to make it clear to me, why would he start flirting again? Does he he like me? Does he just see me as a friend? Does he maybe feel kind of bad for acting distant, and wants to rebuild the friendship?