It is without doubt that khushu (Calmness, concentration, humility and being present minded in Salah) is one of the greatest assets a Muslim can possess in his faith. It comes with struggle, perseverance, and patience and is obtained only by those who work for it, who really set their sights where they should be
Salah (daily prayer) is the greatest practical pillar of Islam and khushoo is required in Salah. Fear of Allah (swt) is the most important cause of khushoo as one always thinks that Allah is always watching over him. Making the heart submissive and humble before Allah (swt) is one of the most important causes of khushoo, because if a heart is corrupted by the insinuating whispers of Satan the acts of the body faculties will also be corrupt. Part of khushoo is to be submissive and humbling oneself out of the fear of God
'Successful indeed are The Believers, those who humble themselves in their prayers'