1. I think the answer to your question is that it is clear from here that when Rav Huna learned Chumash, or presumably any other part of Torah, he looked at the practical lesson that one can one learn from every word. Therefore when he reached "Yairaeh, Yireh" - even though in the time when the Beis ha'Mikdash stood these were happy verses which indicate that one sees, or is seen, by Hash-m - nevertheless Rav Huna was painfully aware that the meaning of these words could not be fulfilled nowadays because later in Sefer Yeshayahu, Hash-m said he does not want us to appear before him.

Similarly when Rav Huna learned the Pasuk that "you will slaughter Shelamim and eat them there" he did not merely look at this as another verse about something that the Jews did a long time ago, but rather he immediately possessed a longing to bring the Shelamim. However he was then reminded that since the destruction of the Beis ha'Mikdash we are unable to do this, and on the contrary Hash-m complained that we brought so many undesired sacrifices.

In other words, even though the Pasuk he read was happy, nevertheless he always thought what lay behind it.

2. When R. Akiva saw the fox coming out of the Kodesh ha'Kadoshim, he immediately thought about what this meant. It meant that a prophecy had been fulfilled that Tziyon would be plowed like a field. Since he saw on the spot that a sad prophecy had been fulfilled he knew that it automatically followed that the happy prophecy - that the elderly and the youth will dwell in Yerushalayim - would also later be fulfilled.

Therefore from the situation that R. Akiva observed now, he already knew that this signified that there would be a happy ending. In contrast, Rav Huna did not yet see the fulfilment of a prophecy, so he could not honestly say that he was happy with the message of the Pasuk he had learned.

KOL YUV

D. Bloom