1)
Why are they happy?
1.
Malbim: Because the ways of Chachmah are Hashem's ways, the base on which the foundations of all the worlds are founded.
2)
Why does it add "Ashrei Derachai Yishmoru"?
1.
Orchos Tzadikim (Sha'ar ha'Ratzon DH Kol Mi): Hashem asks Yisrael only to listen to Him, and then He will fulfill "Im Tovu u'Shmatem Tuv ha'Aretz Tochelu" (Yeshayah 1:19).
2.
Malbim: "Shim'u Li" is accepting the matters; afterwards is guarding in the heart.
