Why is he always happy to see me?

. I just started a service job and one of the guys training me in kept staring at me then looking away when I saw him. I tried smiling, but he was too fast. Then when he went outside he stared in the window, smiled at me, and stuck his tongue out. He likes to joke with everyone, so I'm guessing he's being friendly? I met him first when I went in for an interview and a guy friend came to pick me up. So maybe he thinks I'm taken? The second time I saw him I was training with another and he saw me, stared, I smiled, and he smiled back. When I left he yelled out "it was nice meeting you. You did am awesome job today!"

Then today he calls in saying hell be late to take over my shift and when he realized it was me on the phone his voice got cheerful. When he came in to take my spot he kept looking at me and talking with me as I was trying to leave, then he asked if I was doing anything tonight. I said nothing. He repeated that then added "well doing nothing at home is better than nothing at work" (our job can be slow), and we laughed. We gaze into each others eyes when talking and he stands pretty close and always smiles/laughs and tells me what a good job I'm doing as a newbie. Is he just really nice?
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Or not because tonight he was all serious and not joking like before. He was nice, but.not the same chemistry. Weird thing is he asked me again of my plans and when isaid drinks with a friend, he frowned. Then he mentioned he might go out to with old frat buddies. Then randomly he says "shes hot" in referrence to a girl on a TV commercial. Why do that!? He found out I was in a sorority and when I told him the one he said he has lots of friends in that one...great girls.
Why is he always happy to see me?
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