Why did the shepherd went for the missing sheep?
The Shepherd went for the missing Sheep because of its great value to him.
Firstly, the 99 were well secured with him while the missing Sheep was exposed to danger and attack from Wolves and possibly humans.
Now bringing this down to Jesus Christ, the lost Sheep is a sinner or a backslider who was once in the fold as a Believer.
For those that are sinners and have never known the LORD, they are the lost Sheep. We can term them that because God created everyone but Sin separated humanity from God. After examining this carefully especially the spiritual laws that cannot be overlooked, there was need for blood to be shed- blood of a MAN who has no sin. No Angel or human was worthy except God Himself. So He made Himself manifest in the form of man by being conceived by a VIRGIN through the HOLY GHOST.
Jesus came and shed His blood, died and resurrected for MAN to be saved. The only way to key into this reality is by ACCEPTING and BELIEVING in Jesus.
Now, all Believers are saddled with the responsibility of sharing the Gospel, hence representing Jesus the great Shepherd who leaves the 99 secured and goes after the missing Sheep. We are to go out there, and like Paul said "...that by all means we may save some", we should proclaim the Gospel (JESUS) so that the lost can be found and retrace their steps to JESUS.
God is committed.