Why did Muslims attack the Sasanian Empire in 633?

Map of the Middle East during the 6th century

In the year 633 AD, Abu bakr the caliph of Muslims attacked the Persian Empire. There were three underlying reasons for this attack according to Wikipedia:

"1. Along the borders between Arabia and these two great empires were numerous Arab tribes leading a nomadic life and forming a buffer-like state between the Persians and Romans. Abu Bakr hoped that these tribes might accept Islam and help their brethren in spreading it. 

2. The Persian and Roman populations suffered with very high taxation laws; Abu Bakr believed that they might be persuaded to help the Muslims, who agreed to release them from the excessive tributes. 

3. Two gigantic empires surrounded Arabia, and it was unsafe to remain passive with these two powers on its borders. Abu Bakr hoped that by attacking Iraq and Syria he might remove the danger from the borders of the Islamic State."