R’ Betzalel Zolti zt”l (Ginzei HaSifrei) would say:
"Klal Yisroel sang Shirah; 'אז ישיר משה ובני ישראל' The Yerushalmi (Pesachim 10:6) says that when Hashem performs miracles, the chance to sing Shirah begins when the Geulah is complete. So how did Bnei Yisroel know that the Geulah was complete at the end? It says 'ויאמינו בה'' - the entirety of Geulah is the ability to have Emunah Shleima and that is when they sang.”
A Wise Man would say: “Treat everyone with politeness and kindness, not because they are nice, but because you are.”