Because Jesus Rose From The Dead.
1) In the Resurrection of Jesus Christ, we have hope.
1 Peter 1:3 NKJV
Blessed be the God and Father of our Lord Jesus Christ, who according to His abundant mercy has begotten us again to a living hope through the resurrection of Jesus Christ from the dead,
2) Because of Jesus Christ and His Resurrection from the dead, we have an inheritance in Heaven.
1 Peter 1:4 NKJV
to an inheritance incorruptible and undefiled and that does not fade away, reserved in heaven for you,
3) Because of The Resurrection of Jesus Christ, we have Salvation.
1 Peter 1:5 NKJV
who are kept by the power of God through faith for salvation ready to be revealed in the last time.
4) Because of The Resurrection of Jesus Christ from the dead He brings joy.
1 Peter 1:6-9 NKJV
In this you greatly rejoice, though now for a little while, if need be, you have been grieved by various trials,  that the genuineness of your faith, being much more precious than gold that perishes, though it is tested by fire, may be found to praise, honor, and glory at the revelation of Jesus Christ,  whom having not seen you love. Though now you do not see Him, yet believing, you rejoice with joy inexpressible and full of glory,  receiving the end of your faith-the salvation of your souls.
John 20:28-31 KJV
(28) And Thomas answered and said unto him, My Lord and my God.
(29) Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.
(30) And many other signs truly did Jesus in the presence of his disciples, which are not written in this book:
(31) But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
Jesus is The Resurrection and The Life.
John 11:25-27 NKJV
Jesus said to her, “I am the resurrection and the life. He who believes in Me, though he may die, he shall live.  And whoever lives and believes in Me shall never die. Do you believe this?”  She said to Him, “Yes, Lord, I believe that You are the Christ, the Son of God, who is to come into the world.”