What behavior (or appearance of behavior) would be considered the opposite of standoffish? While not necessary, a single word answer would be very interesting if it is exactly its opposite.
Chummy, or familiar, both having a somewhat negative connotation, as does standoffish.
It depends on what you are trying to say, however.
In the Wiktionary, chummy is defined as “on, or trying to be on, intimate terms”. Sometimes chummy behavior is welcome, or at least describes a welcome state of affairs, e.g. among like-minded relatives at a family reunion.
At other times, though, it can be highly annoying, such as when a telemarketer starts off calling you by your first name, attempting to seem like a friend instead of a huckster.