שמאי אומר... והוי מקבל את כל האדם בסבר פנים יפות (אבות א:טו)
Shammai says... "Greet everyone with a cheerful face"
It is known that while the disciples of Hillel personified the middah of chessed (kindness), and thus generally adopted a lenient halachic approach, the disciples of Shammai embodied the middah of din (the strict letter of the law) and consequently they assumed a strict approach. Accordingly, it is interesting to note, that out of all the Tana'im, the one to assert that a person should greet all people genially is Shammai, the patriarch of strictness. One would have thought that such a statement would be more appropriate for Hillel who was famous for his caring philosophy.
The Baalei Mussar explain that we can deduce from the mishna that the strict letter of the law requires one to greet his friend with a pleasant countenance! A person's face is public property and his expressions affect those around him. No one likes to be greeted with a sour face, and thus to do so would be a lack of proper behavior bein adam l'chaveiro. This idea was pithily summed up by Rav Yisrael Salanter.
One Erev Yom Kippur on his way to Kol Nidrei, Rav Yisrael Salanter asked someone for the time. The fear of this awesome day was evident on this fellow's face and he simply didn't respond to the query. In turn, Rav Yisrael declared, "Why am I at fault that you are a yarei Shamayim?" A person has no right to ignore another Jew because he is caught up in his own personal avodas Hashem.
The essentiality of Shammai's dictum is clear from the amount of effort that our Gedolim invested in acquiring this middah. Rav Wolbe writes that his father-in-law, Rav Avraham Grodzensky (the Mashgiach in the Slabodka Yeshiva), worked for two years on perfecting the virtue of greeting every person with a smile. He didn't have an easy life: His health wasn't good, his wife died when she was very young leaving him to tend for a family of young children, and he suffered the horrors of the Holocaust (during which he was ultimately killed). Nevertheless, he kept his pain on the inside and wore a smile on the outside.
Shammai instructs us, "Hevei mekabal es kol ha'adom b'seiver panim yafos." What does the word "b'seiver" mean? The Meiri explains that even if you are not genuinely happy to greet someone, you should put on a smile so that the person at least thinks ("soveir") that you are happy with their encounter. Even when a person experiences internal turmoil, he should still display a cheerful countenance. Rav Grodzensky personified every word of the mishna.
Moreover, a person's countenance has the ability to affect the atmosphere around him. A dejected look exudes a gloomy feeling, while a smile not only illuminates the room but also energizes those around him.
After living in Israel for a few years, an avreich contemplated moving back to America. He approached Rav Yitzchok Berkowitz Shlit"a and mentioned that he was uncertain as to where the best place is to raise his children. On one hand he felt that his American upbringing had advantages, while on the other hand he understood there are many qualities in the chinuch in Eretz Yisrael that cannot be matched anywhere else. Rav Berkowitz replied, "The best place to raise your children is where you are happy! If you are going to come home in the evening with a sour face, you're going to put a damper on everyone's mood and there is nothing worse for a child's chinuch than that!" This is a pearl of wisdom worth its weight in gold.
A practical application to help implement this idea: When arriving home at the end of a day, take a moment before entering the house to ensure that you have a smile on your face. If you have difficulty doing so after a long tough day, try eating some chocolate. It has a unique ability to put a smile on people's faces, and you might as well exploit it for the sake of shalom bayis, chinuch and proper bein adom l'chaveiro!