Why did he act this way with me?

There was a guy in his 40’s I met in the same office I worked as a intern.I am also in my 40’s too. He was very outgoing flirt and joke with the girls in the office. With me he was very shy so he talk to everyone except me he would flirt but only if we were alone he was really serious so he never joke with me. I am shy myself but I would approach talk with this guy but he never seem to open up. He liked that I flirt back I even hinted to hang out so he liked that I shown a interest in him. But he never ask me out.I would also catch him starring at me a lot from a distance or he would stare deep into my eyes I stare back. He also like stand close to me and brush up against me. He liked that I would flirt back. I do not understand on why he act this way his behavior very confusing flirt one day then cold the next. I am not into mind games he flirt with other girls would look at me, just to see if i was jealous. So then I tried to be friends with him kept my distant even though I liked him thinking he did not like me the same way but kept on flirting and wanting my attention. I added him as a friend on facebook he never accepted my friend invitation which I never could understand why he could not be friends with me. When all I ever did was open up try to get to know him better, flirted to let him know I was interested so I know I did nothing wrong. Wish he would have been upfront told me he was not interested. Maybe it was that we worked in the office only there for 1 year he did not want to get involved or that he was just a flirt and never had any intentions to go out with me. Just curious on why a man his age would act like this.