Friendly, now distant in last few months?

When I started working in my new job, one of my bosses was very friendly, sweet and thought I was an innocent junior worker. I used to think he was weird because he seemed to over care at times, lol. Now, in recent months, he seems to be withdrawing. He's still giving me advice and helping out, but if I speak to him in a smiley, very friendly, chatty way, he doesn't respond. He's not responded a few times. Why is that?