Why do we start off saying Pirkei Avos by first saying the Mishna from Sanhedrin "Kol Yisroel"?
A few answers:
It's to make the simple Jews feel better, knowing that they too have a share in Olam Haba, not just the scholars (Kol Bo, cited in Mateh Moshe 486)
Before studying the maxims for proper behavior in Pirkei Avos, we want to start off by describing the goal towards which these lead us. (R' Moshe Almosnino, cited there)
We recite these chapters in the weeks leading up to Shavuos, as a preparation for receiving the Torah. The first step to that is improving in Ahavas Yisrael (see Rashi to Ex. 19:2). We therefore remind ourselves that every Jew has a portion in Olam Haba, and is not (G-d forbid) beneath us to help him physically and spiritually. (The Lubavitcher Rebbe zt"l, Likkutei Sichos 1:262-3)