Other than his fickle change from Rosaline to Juliet, what are some quotes showing Romeo's short attention span and unreliability?

At the beginning of Act III, Romeo has just married Juliet and joins his friends who just happen to be facing Tybalt and his gang in a discussion about Romeo. Tybalt calls Romeo a Villain, but Romeo says they must not fight because he loves him.

"Tybalt, the reason that I have to love thee

Doth much excuse the appertaining rage

To such a greeting. Villain am I none;

Therefore farewell; I see thou know'st me not" (III.i.58-61).

Romeo cannot sustain this claim for very long, though, because Tybalt and Mercutio do not share his love. Mercutio is so surprised that Romeo won't take Tybalt's challenge to fight that he engages him instead. Once Mercutio is dead, Romeo quickly changes his mind about being a villain and loving Tybalt.

"Now, Tybalt, take the "villain" back again,

That late thou gav'st me; for Mercutio's soul

Is but a little way above our heads,

Staying for thine to keep him company" (III.i.122-125).

Thus, because of his best friend's death, Romeo cannot maintain his first goal not to fight Tybalt and actually kills him instead.

Another time that Romeo seems fickle is when he is talking about his plight with Friar Laurence. He completely gives up on life rather than face the fact that he has been banished by the Prince. Friar Laurence has to call Romeo a sobbing woman in order to get him to settle down enough to tell him of the plan he has to reunite him with Juliet in Mantua.

Then, once in Mantua, Romeo is bound and determined to stick to the Friar's plan until he hears that Juliet is allegedly dead. Again, Romeo forsakes the plan without word from the Friar, buys some poison and kills himself. Romeo is too hasty--too spontaneous. He doesn't think before he acts and this is part of his tragic downfall. He never learns to gather all the information and verify his facts before making rash decisions.