Why did David tell Barzilai to cross with him?
Malbim: The greatest person crosses with the king, for his honor.
Since Barzilai was very rich, why did David need to invite him to eat at his table?
Malbim: This was for Barzilai's honor. This is also why he told him to be in Yerushalayim, the capitol, and not in the small town Roglim.
Why did David want to honor Barzilai?
Malbim (33): (a) He was old. (b) To return good - Barzilai financed David 1 when David was in Machanayim. (c) He was great (wealthy 2 ).
Mishnas R. Aharon 4 p.25: Barzilai gave to David Lechem Nekudim (toasted bread, hard, crumbly crackers or pretzels, or moldy bread - Rashi and Radak, Yehoshua 9:5). Normally this is disgraceful to give to a king, and one is Chayav Misah for this! However, when the king is exiled and fleeing for his life, the smallest gift is worth a great amount. Likewise, at a time of slackening in Torah, any learning has great value and reward. (What is his source that Barzilai gave Lechem Nekudim? Everything mentioned (18:28-29) is dignified, except for barley, which was considered to be primarily for animals (Pesachim 3b). - PF)
Eruvin 86a: Rebbi and R. Akiva used to honor rich people. Aruch - one remains wealthy only if he does Chesed, therefore one should honor the rich.
