My grandfather 88 yrs old keeps on forgetting some peoples around him and events but remember the people he likes the most. it looks very odd.

Not at all odd: Perhaps you did not describe the 'odd' looking part. At his age, over half of people suffer some or significant memory loss. That it is selective is not at all surprising and may even make good sense. If a person (of any age) had their options reduced, they's be wise to choose well and select those they liked best. I hope for you that you are among those he likes. If not, you may be sad about it.