Sheep have little ability to defend themselves, compared with other species kept as livestock. Even if a sheep survive an attack, they may die from their injuries, or simply from panic. When they fall on their back, they could not stand up on their own and they could die in that position. Sheep are so vulnerable. It is interesting to think why the Lord has likened humans to meek sheep.
We are so vulnerable to the things that happen to us, even Christians. One time we feel so strong yet something we do not know would come and hit us so big that we would fall flat on our backs and it seems so hard to recover from the unexpected hard knock. We are vulnerable to sin and temptation. Like sheep, the Good Shepherd knows that we need special care and so He cares for us individually.