The Bitch Switch

So I just started this new job and there's a guy there I find really attactive. The girl that trained me said they were kinda talking but she's not 100% interested and also also I'm dating someone so I wanna keep my distance. We make eye contact all day every time I'm working, but the most I ever said to him was thank you. I smile and I'm super friendly to everyone there but whenever he comes around we make eye contact and I look away or don't acknowledge him at all. I roll my eyes, I don't say hello or smile or anything like I do to everyone else. It's automatic, I'm just cold asf to him. I feel like I'm being rude but idk how to stop it. What is wrong?