See R. Ovadiah Barternura's commentary.
While he states the general focus of the Mishnah itself implies social relationships, and in this Mishnah, he states that when one invites someone to his home (a "social" function), he should not keep his face in the ground and have an "embarrassed look", regarding this last phrase of "greeting everyone with a cheerful countenance", R. Bartenura explains that this means that one should conquer his evil inclination and conquer against evil thoughts in his heart - which is a self-disciplinary function, per-se.
As a general rule, the written Torah does not provide much direct instruction regarding personal character directives, such as "Be a social butterfly; not a hermit". Perhaps, this is where the main focus of Pirkei Avot comes along to "fill in the gaps". I think Rabbi Yonah and Barternurah commentary on Pirkei Avot's 1st Mishnah which states "Moses received the Torah at Sinai" delve into some detail on what the term "Torah" means, and eventually, explain something about how Pirkei Avot, which deals with ethics, is part of the Torah. I think, there they explain, a bit, as to why the Torah does not, itself, explain directly about ethical behavior. Contact me if you have trouble locating this.