I'm dating and I don't even know it

My co-worker just told me he thought I was dating someone. He said he's been getting that impression from me for a while now and just didn't want to be nosey and ask. This is very interesting considering the fact that I am not dating anyone. I don't know how it's possible to "give the impression" of being in a relationship, but apparently that's what I have done. He seemed genuinely disappointed when I told him no boyfriend was in the picture.