@Shikuretto_Tsuki
Because he is fun to her and she admires his art. She sees he's a good artist and has much potential and drive to improve.
As you see in this chapter, she is popular with the guys. She is used to boys chasing for her affection either by trying to act like Chads or braggarts. The one boy in this chapter acted like he was such hot shit for creating sub-par music and it's this sort of behavior she always receives.
With Senpai, it's clear he puts actual work into his hobby. On top of that, he's humble about it; like he doesn't realize it's good. He's also not the type to chase after her like everyone else does, so he's safe to her. She's the one who wants to do the chasing, albeit in her own kind of way. And he likes to run away from her, metaphorically speaking.