Consider why the Israelites were to select the lamb on the 10th but not sacrifice it until the 14th. They were to select and separate the lamb from the flock in order that it might be observed to make certain it was fit for sacrifice, or truly without blemish. But to do this someone in the family, and possibly several members of the family, would have to spend time with the animal and could become attached to it, almost like a pet. It would no longer be considered simply a lamb but their lamb. Notice the wording used in Exodus 12:3 is “take a lamb” but in verse 5 (of the ESV, NASB and King James translations) Moses calls it “Your lamb.” I believe that it was important that they grasp the fact that there was a cost involved with the sacrifice: an innocent life was taken so that they could be passed over by death. A death occurred in every home in Egypt that night, in the Egyptian homes it was human and in the Israelite homes an animal, but a death all the same.