Let us remember what has just happened prior to the beginning of this hilarious comedy. Don John has just tried to overthrow his brother and the soldiers that enter in Act I scene 1 have just fought the rebel force of Don John's soldiers. Don Pedro, Don John's brother, has forgiven Don John and receieved him "into his graces," yet clearly Don John is overwhelmed by bitterness because of his failed attempt to seize power. In addition, let us remember that Don John is the bastard child of his father, whereas Don Pedro is the legitimate heir. Bastard children had no hope of ever inheriting, and were always doomed to be regarded as being lower than legimitate children. Thus we have many reasons for Don John to be overwhelmed by bitterness. However, let us also remember what Don John himself tells us in Act I scene 3:
I had rather be a canker in a hedge than a rose in his grace. And it better fits my blood to be disdained of all than to fashion a carriage to rob love from any. In this, though I cannot be said to be a flattering honest man, it must not be denied but I am a plain-dealing villain.
So, in addition to his status as a bastard and his failed revolt, Don John also chooses to be a character that gives in to negative thoughts and is overwhelmed by bitterness.