Why do guys do this?

Okay, so here’s my question: I’m a librarian. I’m 24, cute, single. I’m also pretty shy. I met a guy in the library. He’s nice. He’s single (he told me). He’s the right age-ish. He’s not shy. He keeps coming by to see me, a few times for help in the library, but often just to “say hi”. We laugh a lot, he’s touched me casually, we talk about all sorts of things; he’s asked me what my interests are and we have a few in common, but he hasn’t asked me out. So my question is, “why?” I mean, I know you can’t read this particular guy’s mind, but guys, especially; can you enlighten me as to why you wouldn’t ask a girl out under these circumstances? I’m really shy, and I’ve tried to be as flirty as I know how to be, but is it possible that he still doesn’t think that I’m interested after four 45 minute long conversations? I don’t have much relationship experience, but this seems like this should go somewhere, and, well, it isn’t. Any thoughts?