I don't get this. Why does the sensei like the student?

Because he's cute and tsundere as fuck.

I think sensei found Ikuta's strong and honest feelings (For the girl) and everchanging expressions appealing and cute.

And how although Ikuta seemed to love her so much, he was unable to say it straight to her, and sensei got curious of Ikuta's tsundere nature. And when they started to spend more time together, sensei just came to love for Ikuta.

Amen