The Midrash describes the Jewish people as God's "sheep" (Shir Hashirim 2:16), as an expression of their total dedication to God, like shhep that follow their shepherd unquestioningly.
Ya'akov exemplified this level of dedication while he worked for Lavan, remining loyal to God's commands despite the spiritually alien enviroments. Thus, to hint to Ya'akov's dedication, God rewarded him with wealth that came about through amassing sheep.
However, when Ya'akov returened to confront Eisav, he did not stress sheep as his most important acquisition, but rather, oxen (32:6). Ya'akov was hinting, "Because I am dedicated to Gad like a quiet sheep, therefor I have God's might behind me, so I will be as strong as an ox against you!"
This teaches us that, in our observance of mitzvos we should be utterly humble towards God like a sheep; but when fighting the forces that oppose Judaisam in the outside world, we cannot stand by sheepishly and watch Jews be drawn away from their heritage. Rather, we must fight for Jewish values with the strength of an ox.
(Based on Likutei Sichos vol. 15, p. 25ff.)