why did he rise again?

Jesus was both God and man in one - God the Father cannot even look upon sin - when man sinned, the bond/Oneness with his Creator was broken - God could have allowed man to die with no-hope, no-chance of fellowship with Him in Heaven - but He Loved us so much He made a WAY…”Jesus” - there had to be a way/method/a sacrifice by which man’s sins could be forgiven…Jesus paid the price/sacrifice for us on the cross - as He said on the cross, “It is Finished” - our sins were covered by His sacrificed blood - now whosoever believes in Him shall not perish but have Everlasting Life (John 3 : 16)

Note the word “shall” means manadatory, not maybe. http://www.jesus2020.com/video.html

Salvation - a gift offered freely.

“God gave Jesus to die for our sins, and He raised Him to life, so that we would be made acceptable to God” (Romans 4:25)

The idea of a god rising from death is as ancient as history itself. Virtually all ancient religions were rooted in this concept, including Christianity.

It was to confirm the prophecy that on the third day he would rise. There were many specific prophecies said about the messiah this was one of them. There are two main reasons for this, the first one is that his ressurection would mean the resurrection of the temple as well. The second was so that everyone knew beyond the shadow of any doubt that he has fulfilled the prophiecys needed in order to be the messiah.

jesus died to show us that there is Resurrection after death