We are up to Mishnah 16 in the third chapter of Maseches Avos (in some versions of the Mishnah it is counted as Mishnah 12). Its words were said by Rabbi Shmuel, who lived after the Destruction, the author of the Mechilta, the halachic Midrash. Following the previous Mishnah, which dealt with the prohibition of embarrassing people, our Mishnah comes to teach how a person should relate to others.
The words of the Mishnah are extremely brief, and their language is somewhat obscure. For that reason they are open to many interpretations as to their precise intent. We will follow here the approach of the Bartenura, which is sufficient for our purposes.
The words of the Mishnah:
"Hevei kal l'rosh, v'noach l'tishchores, vehevei mekabel es kol ha'adam b'simchah" - literally: be light with the head, be easy and calm with the "tishchores", and receive every person with joy and gladness.
"Hevei kal l'rosh":
The "head" that the Mishnah is speaking about is the person who is worthy of honor, the one whose place is at the head, such as in the beis midrash, the elder and the senior figure. Toward him a person is required to be "light," meaning quick and prompt in serving him: if he says something, one hurries to carry it out. This is the proper way to relate to the senior person, the one who is fit to sit at the head.
"V'noach l'tishchores":
"Tishchores" literally means the one who is blackened, the one whose hair is black and dark and has not yet begun to turn gray, that is, the young person. Toward him the quality of "noach" is required. According to the Bartenura, this means "with calm and settledness": composure and deliberateness. When the young person asks for something or says something, one is not drawn after him immediately and does not rush to fulfill his wish, but rather weighs the matter, arrives at the most settled decision about it, and only then responds accordingly. This is conduct of clear-headedness and deliberateness in dealing with those who are not senior.
"Vehevei mekabel es kol ha'adam b'simchah":
Toward everyone - young and old, elders and children, and even Jew and non-Jew alike - a person is required to be "mekabel." The word "mekabel" comes from the root meaning "opposite," as when a person stands facing his fellow: his expression and his attitude toward him are with joy.
Shammai said in his Mishnah: "Hevei mekabel es kol ha'adam b'sever panim yafos" - a cheerful appearance and a smiling face. But Shammai's words are speaking mainly about the external: there is no requirement that a person actually feel joy, as long as he is pleasant and does not treat people unpleasantly. Our Mishnah adds and says "b'simchah" - with genuine, inner joy, and that is a great matter.
The finest example of this is a mother of the sort one occasionally merits to know. When someone calls her on the phone, she says hello as though she had been waiting for that call all day; when she walks into a room, she greets everyone as though she had been looking forward to seeing them all day. And all of this comes from a genuine smile and sincere joy. Such conduct has an enormous effect: sullen people soften, ordinary people are uplifted, and they automatically feel affection toward her. More than that, they receive real energy, which draws out the best in them.
In summary: The Mishnah teaches three principles in how we relate to people: alacrity and lightness in serving one who is greater and older, calmness and composure in dealing with one who is younger, and receiving every person genuinely and with inner joy, not merely outwardly. A person should approach every person out of sincere gladness to speak with him, to meet him, to help him and to be part of his day. This has the power to draw out the good within him, and to make the world a better place.