Why did the Companions, who were given glad tidings of Paradise, fear Allah with the utmost fear, despite the Messenger's tidings to them of Paradise, and why was the Messenger, Allah's peace and blessings be upon him, himself the most fearful among them?

The Companions are the best of generations and the most excellent of the Ummah in every virtue. It is from the perfection of faith to worship Allah with fear, hope, and love combined. The more a servant increases in faith and knowledge of Allah, the more their fear of Him increases, as was the case with the Prophets and the Companions. Their fear stemmed from their understanding of the meaning of worship, their knowledge of Allah, and their pursuit of more reward.