Among the blessings that Yakov gave to his sons before his death, was the unusual Bracha given to Yissachar. He was compared to a donkey that shoulders a heavy burden.
It was well known that there was an arrangement between Yissachar and Zevulon where Zevulon shared in the reward for the Torah study of Yissachar by supporting them financially. To this day, people make a Yissachar-Zevulon arrangement.
The של״ה הקודש says that the donkey is chosen as the symbol of Torah study for several reasons. The donkey is humble and undemanding. It sleeps in uncomfortable quarters. The donkey is a beast of burden. When one accepts upon himself the yoke of Torah, the yoke of government is removed from him.
One who studies Torah is revered in the eyes of G-d. Torah study expels forces of destruction. We must be prepared to take upon ourselves this burden of Torah, so that all other burdens be removed from us. Shabbat Shalom