Why is Muhammad called the seal of the prophets?

Muhammad PBUH is the seal of the prophets & this made the message of Islam the last universal message from the Almighty God to the all mankind & the Holy Quran as an everlasting miracle among his miracles PBUH which addresses the hearts & the minds of people in all feilds. Muhammad PBUH assured the message that all the prophets who proceeded Him came up with which is the Monotheism; to believe & worship the only one God with no one associated to Him.