This question makes it sound like you think God was obligated to send His Son to die for us. You must understand that this is not the case. God could have chosen to not offer a way of salvation and sent us all to hell with no chance of getting to heaven.
But that's where God's love stepped in.
- Romans 5:8
But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
This verse says that even while we were still dead in sin, God loved us so much, and He demonstrated that love for us by sending His Son to pay the penalty of sin in our place. So the answer to your question is: God loves us; that's why He sacrificed for us.
If your question is deeper than that (i.e. why did God have to sacrifice anything at all? why not just forgive us all without having to make a sacrifice?), the Bible gives us an answer for that also.
- Hebrews 9:22
And almost all things are by the law purged with blood; and without shedding of blood is no remission.
According to this verse, without the shedding of blood, there can be no forgiveness of sins. And only the blood of an eternal God (made manifest in Jesus Christ) can pay the eternal penalty of sin. So in order to be able to offer salvation to mankind, God had to offer His Son as a sacrifice in payment of the penalty of sin.