Whose youths will be pleasing in your eyes?
Rashi: Mine (David's), when your servants will tell you about them.
Why did he say "Al Yom Tov Banu"?
Rashi #1: We relied on your feast; we came now to ask. Radak - since you are making a feast for your shearers, you need not spend more for us 1 , since you prepared much food and drink.
Rashi #2 (from Midrash Shmuel 23:12): (It was Erev Rosh Hashanah.) We need food for Seudas Yom Tov.
Malbim: It is proper to be gracious to everyone who asks on Yom Tov
David had 600 men. If Naval prepared a generous Seudah for 100, e.g. twice as much as needed, and gave the extra to David's men, each would get a sixth of a portion. Below (36), Malbim says that Naval did not notice that anything was missing! (PF)
What is "Asher Timtza Yadecha"?
Malbim: According to your ability and generosity of your heart.
