God wants His children to know we are His children. Think about it. Only a bad father would not want his children to know who their father is. God is not a bad Father! He is not only a good Father, He is the perfect Father.

He wants His children to have assurance, not only so we will know we are His children, but there are many blessing associated with having assurance of salvation. Here are seven (there are no doubt more):

Assurance affords a person joy instead of doubt and fear of damnation. Romans 8:15
Assurance affords a person boldness in evangelism and missions, since he knows that the gospel he proclaims is true and that God will save His people. 2 Corinthians 5:18-21
Assurance gives power and confidence in prayer, since the person knows that he has a relationship with God and that God hears and answers his prayers. 1 John 5:14-15
Assurance promotes worship that springs from a truly grateful heart. Colossians 3:16
Assurance encourages holiness and obedient living motivated by gratitude for salvation rather than as a means to earn salvation. 1 John 3:6-10
Assurance helps a person face trials, knowing that his Heavenly Father who loves him is in charge of all things. Romans 8:17-18 & 28.
Assurance gives Christians peace when facing death. Revelation 14:13, Isaiah 57:1-2