why does he treat me like this? (15f, 17m)

im a sophomore, abt to turn 15.  theres this pretty popular senior at my school 17m.  we used to sit together in one class.  he always said hi when i arrived at that class.  one time, i was absent and he told me later that he missed me.  our seats got switched a week ago.  today, it was lunchtime and i was walking on the street in front of him, and he said "(my name), great to see you".  i replied in a tone that i now realize mightve come across as sarcastic (i speak in a very monotone way usually), "great to see you too."  i highly doubt he's being sarcastic because he doesn't smile or laugh, and his tone seems serious, and it fits with his personality for him to say this stuff.  why is he like this towards me?