Is it normal that this manager says hi one day and the other not?

There is this manager at my work who is 27 , he is not my manager but I see him everyday . Since the first day I came to work he has been staring at me and used to come to take papers from me without a reason . We started saying hi but one day he says hi to me in a nice way and the next day he ignores me and frowns while passing by me . He also used to wave and smile at me while talking on the phone but he stares at me so much and I didn't want to stare back because I was new at work and didn't want him to think I am an easy girl . I respect him but i don't return his stares back because I am worried he is playing with my feelings . At events inside my work he tried to come and participate when I was standing with a group of people but the conversation ended once he came and a girl embaressed him and asked him why he is standing with us . Also when I laugh with other boys he looks with a sad face . Two times he ignored me purposely and didnt reply back when I said hi . But then the next he was trying to approach me and looking at me and I ignored him. Few days ago I did something good related to his work and he told me good job and sent me an email thanking me and encouraging me but the next day I caught him staring at me 2 times and once I saw him he turned his back to me and then I passed by him and he frowned and didn't want to look at me to say hi . Why is he doing this . Is he playing with my feelings? I like him too but I don't understand him