Why does male coworker act flirty/friendly then completely distant?

sadiefluff
I need some insight: I recently got hired at probably the first job I’ve ever actually loved and I was lucky to get, working at a middle school as an aide to two male teachers. I am an attractive female (30’s) about 11 years younger than my male coworkers. All three of us are married and have mentioned our spouses. One of the teachers (the one I work with more often) is very attractive to me and I have felt serious sexual tension around him and attraction towards him (I have zero clue if he feels that way back). However I love my husband and have zero intentions of ever cheating on him. However I have noticed that this guy is seriously a charmer and has a flirtatious, and extremely likable personality. Some co-workers have called him “shy” but I actually find him very social, reserved, yet playful/flirty. And it is frustrating for me because of my attraction towards him. Lately he seemed to to be a bit more flirty than before: Playfully pulling the napkins away from me in the break room so I couldn’t reach them, joking with me and teasing me. When we talk, we both hold eye contact a lot more intensely than I think is normal, often standing closer to each other at times than I feel is normal etc. And then there’s days where he seemed flustered and nervous around me. However, he then will come in and suddenly be super aloof towards me, almost cold, like I’ve done something wrong and he doesn’t even like me. My other male coworker has expressed noticing that as a friend he can go hot and cold towards him too. I feel jerked around. I just want to have a professional yet friendly relationship with him. But his playful flirty then cold removed back and forth has me frustrated and extremely confused. I want to keep this job as I love it. And it is not an option to avoid him at work because I am literally his aide and work side by side to him. Why is he acting like this?
Why does male coworker act flirty/friendly then completely distant?
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