"... All we like sheep have gone astray; we have turned every one to his own way..." (Is 53:6 RSVCE). Through our sin, pride and selfishness, we are like sheep who have gone astray from the Lord. 

"...I myself will search for my sheep, and will seek them out" (Ez 34:11 RSVCE). The Lord states that he will be our Shepherd and will come after the lost. He takes the initiative to seek and save the lost. 

"I am the Good Shepherd. The Good Shepherd lays down His life for the sheep" (John 10:11 RSVCE). Jesus is the Good Shepherd, and the way He seeks us is through His love; the supreme act of sacrifice for our redemption. The oracles of prophets are fulfilled in and through Jesus. 

In the parable of the Lost Sheep, the shepherd risks everything to go in search of the lost sheep. To leave the ninety-nine sheep, which would be his entire livelihood, in search of the one lost sheep, would have been ludicrous. It made more sense to write-off the lost sheep, but God is willing to risk everything; sending His beloved Son to offer Himself in sacrifice so that we might have life and have it abundantly. 

Once found (faith and repentance), all of Heaven rejoices!