The answer you are looking for is given to us in Act I scene 1. In this scene, we see Orlando confronting his brother over the alleged abuses that he has suffered at his brother's hands during his childhood. We see Orlando's keen resentment at the way he has been treated by Oliver when he wrestles with Oliver and, when he has Oliver in a hold from which he cannot break free, accuses him of the following behaviour:
You shall hear me. My father charged you in his will to give me good education. You have trained me like a peasant, obscuring and hiding from me all gentleman-like qualities. The spirit of my father grows strong in me, and I will no longer endure it! Therefore allow me such exercises as may become a gentleman, or give me the poor allottery my father left me by testament; with that I will go buy my fortunes.
Thus we can see that Orlando thus resents the way that his brother has deliberately ignored their father's will, which instructed Oliver to ensure that Orlando received a good education that was appropriate to his social standing. Oliver, however, brought him up as a "peasant" without allowing him to develop the "gentleman-like qualities" that were Orlando's birthright. This is the cause of the resentment that he feels towards his older brother.