The avenging of the Persian invasion was a factor but ultimately was probably more propaganda and pandering to the peoples of the Greek mainland whom he had recently conquered and was trying to gain support from. Most Greeks form the peninsula didn't even consider the Macedonians to be "Greek" and Philip was determined to legitimize his rule. As for his real motivations to conquer Persia, he had already shown an imperialistic streak in his conquest of Greece and his campaigns in Thrace and Illyria naturally Persia would seem to be a great challenge to the man who, at the time was the greatest general in Greece and likely the whole known world. Honestly had it not been for his son and his own assassination, Philip likely would have gone down as a much more prominent figure in history and would be regarded as one of the greatest generals of antiquity. Ultimately, his greatest motivation for invading Persia would be the treasure that was to be which was more than he could have imagined when one looks at how wealthy his son Alexander became when he conquered Persia.