The third of Rabbi Yishmael’s ways to maintain peace of mind:
3 — Receive all men cheerfully
Elsewhere in Pirkei Avot, Shammai says, “Receive all people with a cheerful countenance.” 
On Rabbi Yishmael’s mishna, Rabbi Hertz  refers back to Shammai’s earlier mishna:
“Have a friendly manner in all your dealings with [people].” 
Rabbi Yishmael’s statement echoes Shammai’s — a sign in itself that even then, this was a part of Torah that was transmitted from one generation to another.
Even stronger is the Rambam’s statement:
“…you must receive every [person] — small and great, free man and slave, every sort of person — happily and in good cheer.” 
What distinguishes Pirkei Avot from the strictly halachic (Jewish law) parts of Jewish tradition is its emphasis on practical application. For the most part, PA concerns itself with things we do, rather than with things we contemplate theoretically or argue about unendingly, except insofar as that contemplation accompanies or underlies the things we do. Almost every part of the Talmud is about what we do; PA is about things we do informally, in the ordinary course of daily life.
And yet, in making this emphasis, PA points to the essence of Torah itself: These things, too, are Torah. Our behavior towards others is worship, too. Torah isn’t a work of “philosophy.” It’s not an abstract academic discipline. Nor is it about unreachable ideals that are uplifting to muse upon, but which no one expects ever to realize in ‘reality’.
Torah’s about daily life itself. In particular, it’s about the individual relating him/herself to the wider Creation in and around him or her:
“Torah study is…the application of the Torah’s principles to real people and real life situations… [how] the Torah’s eternal truths apply to the human condition.” 
A radical expression of this is the report of Rabbi Mordecai Kaplan reading from Ahad Ha-Am or John Dewey, wearing tallit and tefillin:
“[Ira Eisenstein] observed Kaplan praying daily, although sometimes he seemed to use texts other than the siddur, the standard prayer book. His other favorites…were works by Ahad Ha-Am and John Dewey…[Kaplan was] a deeply committed Jew…donning his tallis and tefillin..and then davenning from Dewey.” 
In effect, Rabbi Kaplan was saying, “These works are Torah, too.” Or, put another way, he was demonstrating his intent to view these works in the same spirit with which he views Torah.
(I wonder if a similar case could be made for contemplating a work of fine art, or of listening to a musical piece? It might be argued that creating art is even more revelatory than perceiving it, but doing so would be difficult when wearing tallit and tefillin.)
Commentaries on PA can themselves be sources of insight and important guidance in dealing with human interactions:
“The present mishna [3:12 or 16, depending on the edition] deals with ego difficulties relative to communal functioning. Primarily, they may be said to focus around individuals who have not reached the position of prominence in the community they felt was appropriate for them. The general tendency of such individuals is to downgrade those who have superseded them and to discourage those who would in the future gain the very positions they have failed to attain. The advice of this mishna is to conquer this ego difficulty, not to destroy the structure of community. In other words, do not envy or covet another individual’s position and show envy by not co-operating with leadership.
Rather, be amenable with a superior. Do not ask why that individual is better than you and why you should follow. Rather, say that this is a position of responsibility, and the only way that responsibility can be realized is if the public co-operates.
Then there is the matter of the attitude to those who are on the way towards gaining positions of prominence and becoming superiors, the youth of the community. Do not let the disappointment and frustration with your stunted growth lead you to discourage those who are on the way up the communal ladder. Your melancholy at not having made it should not be employed in discouraging those younger than you who will realize and achieve what you failed to achieve. Rather, be co-operative with youth.
Finally, receive all people with cheerfulness. You may have a chip on the shoulder from what you perceive has been denied to you. Nevertheless, it is up to you not to spread your melancholy to others and…deny them the excitement that they feel towards their work or towards life in general.” 
To explain Torah-study, think of learning to play the piano. To do so requires more than reading about the piano, or reading about music, or even reading about learning to play the piano. To do it, you have to — do it. You have to face the challenges in doing it, improve skills (especially dexterity and brain-hand coordination). To do it really well, you have to go beyond the purely mechanical elements altogether (without ignoring or avoiding them), to invest your playing with real feeling and appreciation of the music. To be a really seasoned piano player, you also have to become comfortable playing in front of people — which usually involves overcoming fear, self-consciousness, shyness, perfectionism, etc.
“Being Jewish means being a mentsch [lit. ‘a man;’ i.e. a mature adult] in all circumstances.” (Rabbi Yisrael Rutman) 
Like it or not, part of being a “mentsch” means “receiving” all people cheerfully by internally looking on all people positively:
“Irving Bunim in his [book] Ethics from Sinai points out that the Hebrew text [PA 1:15 (Shammai) and 3/12 or 16 (Rabbi Yishmael)] actually reads, ‘kol ha-adam‘. Although the standard translation is ‘every man,’ the literal translation is, ‘Greet the whole man [i.e. person] with a cheerful countenance.’ ‘In other words,’ writes Bunim, ‘if there is something about a person that displeases you, look at the total personality and find his good qualities; then you will be able to greet him with a cheerful countenance’.” 
Every facet of Torah is essentially worship. As such, it means that every facet is about our relationship with G-d. The peace of G-d is already with us, as G-d fills all Creation. Our role is to avoid disturbing the Peace that has been graciously given us, just as Patanjali observed: Yoga (or worship or devotion or d’vekut) is restraining the constant turbulence of the finite mind, in order to allow the peace of the Infinite Mind in us to express itself.
Torah-study should be for us a gradually higher and deeper practice of daily life itself, in which we become better and better people inwardly and outwardly.
Rabbi Yishmael points the way to do this.
“Receive” is קבל in Hebrew. From the same root comes the verb “to accept.” Therefore, to “receive” all people cheerfully means to “accept” them as they are.
 Pirkei Avot 1:15
 Hertz, Rabbi Joseph; Sayings of the Fathers; Behrman House; © 1945; see pp. 24 & 58
 Goldin, Judah, trans. and ed.; The Living Talmud — The Wisdom of the Fathers; Mentor Books; © 1957 by Judah Goldin; p. 137
 Rabbi David Rosenfeld;
 Rabbi Scott Meltzer, quoting Ruven Bulka (As a Tree by the Waters):