How do I show my interest?

I started a new job in December, I like a guy who works in my building, he works on the floor above me and we both do totally different jobs so there is no reason our paths would cross work wise. When we ped in the corridor or on the way out of work he was always super friendly, I am initially a little shy when I meet new people so usually it was a quick Hi and a smile. Recently though hes not been as friendly when i've seen him, and I get the feeling its because hes umed i'm not interested. This is not the case, I don't know what to do. Please remember I am shy to begin with but come out of my shell the more I get to know someone so find it difficult to show I like someone. Advise please. I'm in my 20's.