Warning:This contains a lot of boring context so read at your own risk!

I fell in love with Stefonnie way back in season 1.
It is my OTP after Bamon.They are so cute together and have undeniable chemistry.
It's one of the reasons I believe the writers are keeping Bonnie away from the Salvatore(the chemistry)!

Stefan and Bonnie from the very first episode 1x01 Pilot had a connection.Bonnie noticed him first and if my memory serves me correctly said "that's a hot back",indicating quite clearly that she found him as attractive and judging from his smirk in the classroom that attraction is reciprocated,lol!

What happened to that attraction I wonder?

I personally have always thought that it is still there but they both buried under the surface for the same reason = Elena.
They are both far too loyal IMHO to ever cheat with each other behind a Elena's back(in any shape or form)so the easiest way apparently for them is to hide this attraction and pretend it isn't there.