Taught Shamai - Greet everybody with a happy face - "הוי מקבל את כל אדם בסבר פנים יפות" (Avos 1:6). Notice that it was Shamai, whose neshama comes from the world of strict judgment, who taught this halachah and not Hillel, although Hillel is famous for his kind nature. This teaches us that even according to the strictest interpretation of the Torah, one should greet everyone with a pleasant and cheerful countenance.
Reb Meir of Amshinov zt'l would say that הוי מקבל את כל האדם בסבר פנים יפות also includes oneself. One should accept himself the way he is, with joy and happiness. He shouldn’t harp on his faults and feel dejected.