Q. Why were the dogs in parshas Bo rewarded with only scraps of treif animals, while the donkeys were honored with the holiness of kedushas bechora?
A. Horav Yosef Chaim Sonnefeld is quoted saying that at the time of Yetzias Mitzraim, the dogs merely kept quiet. The donkeys however. shlepped and carried. When you work hard to help another, you are invested with kedusha. Putting down your shoulder to help someone else is a higher level of spirituality then merely keeping quiet.
Horav Shlomo Miller's Shlit'a opinion is similar. (See That is on Parshas Bo 79, for a different answer).
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a