You were a 5th year, and he was a 7th year.

and he protected you like you were his.

Gryffindor was celebrating their most recent win over Ravenclaw

and you were invited to the party despite whatever your house was just because you were close with Ron and Harry.

so when you showed up you took a seat on the couch in the common room

a couple Gryffindors came up to you and flirt or try to get you in their dorms.

and Ron, being the protective lion he was.. was not having it.

he was about to beat someone ass.