Someone please explain?!

About a month ago i went out on a date with one of my coworkers. We went out twice and both dates went great. Nothing was decided that we would go out again but i kinda of just assumed we would since it went so well. Well, now it seems that he has become distant and he won't talk to me that much anymore. However I do notice that he goes out of his way to walk past me and sometimes he acknowledges me and sometimes he doesn't. I'm confused and a little sad because we had a really good work and personal relationship before. Why does this happen?

Just to help narrow those choices down some: You say you went out with him TWICE. So who was it proposed the second date?