Most Helpful Guy
He could be day dreaming... I awkwardly sometimes make eye contact with strangers when I day dream without knowing... OORRRR he could interested in you and too shy to say anything about it... think of Leonard from The Big Bang Theory... It's not uncommon.1
Most Helpful Girl
Could be many reasons...
1. Day dreaming and you happen to be there.
2. Not into you, just judging you in his head but doesn't want you to know that.
3. Is into you but doesn't want to admit it and seem like a stalker.0