Why wouldn't a guy respond to your text message?

Theirs this guy that I know, He flirts with me all the time always complimenting me and tells me how good I smell. We really don't talk or text over the phone, But I notice if I ask him something through text he doesn't text back. But if he does it's really short, I notice he would call me immediately and follow up, Or he will wait until he's back in town and bring it up than. For instanced I was talking to him about his friend which I am dating , And he didn't reply. He lives far from us, So we don't see him that often. He immediately asked if I missed him , And begged me to come see him of course with my boyfriend. He did tell me once I can't believe you want him. But I never told my boyfriend what he said. But why wouldn't he respond through text, I only text him like twice a year. And that's only to tell him happy birthday or if my boyfriend in I argue about something I might ask him to talk to him. But again were all friends. But when I talk to my boyfriend he tells me let's call him bob. He will say bob asked about you and wanted to know how you were doing. I just don't get his behavior.