Why do you feel guilty even though you're the one who reject them?

Okay so I'm just an unpopular, quite, introvert girl who just like drawing and at that time I'm just a freshman at highschool (14 y/o, in my country, age 14 is already in highschool) . One day that 'guy' (one of my classmate) saw me drawing anime shit then we kinda share interest. We're kinda animemates? We usually talk about our interest untill years past....we got separated, we were not in the same classroom (16 y/o). Then we never talk since then, just sometimes when he go to my class bcuz he wanna talk with his bros.. so we just greet each other. And things not really happen.. until this year (I'm 17 y/o rn) just a month ago, he suddenly texted me. Asking casual thing, "what r u doing" "what r u up to" and I just reply them like what friend should answered. He kept chatting me eventho I never chat him first until yesterday, the day he finally confess and he said it was since we're 14 y/o. I'm not surprised at all bcuz it is obvious but tbh idk how to react bcuz I think I just like him as a friend.. nothing more.. And after he confessed, I feel guilty to answer "no" cuz I know I will not get to befriend him anymore after this but don't wanna say "yes" cuz dont wanna date him (I know I'