Why did he say "you requested a difficult matter"?
Rashi: I cannot give to you more than I have!
Radak: It would have sufficed to ask for one share of my Ru'ach, or half a share.
Why does it depend on being able to see Eliyahu when he is taken?
Rashi: It is a sign that I can give to you more [than I have].
Radak: If you will have Ru'ach Elokim to see me then, I know that Hashem wanted you to have like you requested.
Malbim #1: According to my Perush that he wanted Eliyahu's Ru'ach to rest on him, now Elisha was like the moon, which receives its light from the sun. He wanted to be his own source, like the sun. This will be only if Eliyahu totally leaves the world, but not if he keeps a connection to it. When Moshe died, Zekenim said that [Moshe was like the sun, and] Yehoshua was like the moon (Bava Basra 75a), for Moshe did not totally depart, and Yehoshua still received from Moshe's form.
Malbim #2: Refer to 2:9:2:1*.
