Why do you think The Giver gives Jonas the memory of family rather than keeping it longer?

The Giver wants Jonas to understand family..... with the loss of his daughter, and his wife living in another place, the Giver's family isn't with him. He is the Giver.... and as the Giver he is isolated. In his own words, the Giver has his own memories..... and we know he loved his daughter. The Giver experienced love, thus, the memory wasn't so hard to part with.

"I will. I am left with a vague wisp of that one; and I have many other memories of families, and holidays, and happiness. Of love."