Grandpa likes iron skillets but not aluminum pans
He likes m&m but not just "m"
He likes Michigan but not Georgia
Why is that?
likes the number 26, but superstitiously dislikes the number 13.
The three examples:
Iron has atomic number 26, while Aluminum's is 13.
m&m's letter positions sum is 26, and m is the 13th letter of the alphabet.
Michigan was admitted into the US as the 26th state, and Georgia was the 13th of the colonies.
With only three examples, this risks being too broad... One possible solution is that Grandpa likes:
Words/phrases with exactly two "holes" in them.
iron skillets (2) vs aluminum pans (3)
m&m (2) vs "m" (0)
Michigan (2) vs Georgia (4)