Final answer:
Claudio's negative reaction to Don John's news about Don Pedro marrying Hero shows that he is easily influenced and lacks trust in his friend.
Step-by-step explanation:
By reacting negatively to Don John's news about Don Pedro marrying Hero, Claudio shows that he is easily influenced and lacks trust in his friend. His immediate reaction without seeking more information or talking to Don Pedro suggests that he is quick to judge and jump to conclusions. This reaction may be motivated by his own insecurities and fear of being betrayed.