Why does Romeo ultimately say that he will go to Mantua?William Shakespeare's "Romeo and Juliet"

In "Romeo and Juliet" after an enraged Romeo slays Tybalt in an act of revenge for the death of his dear friend Mercutio, he is banished from Verona by the Prince who declares,

Let Romeo hence in haste,/Else, when he's fuond, that hur is his last./Bear hence this body, and attend our will./Mercy but murders, pardoning those that kill. (IIII,i,177-180)

After his act of murder, Romeo retreats to the cell of Friar Laurence; he asks the Frair "what news? (III,iii,4) and when the priest informs him of his banishment, Romeo decries it as having more terror than death.  The friar seeks to comfort him, saying that "the world is broad and wide" (III,iii,16), but Romeo contradicts him,

There is no world without Verona walls,/Buit Purgatory, torture, Hell itself./Hence banished is banished from the world,/And world's exile is death. (III,iii,17-20)

The Friar attempts to change Romeo's desire to commit suicide by giving him "armor to keep off that word":  philosophy, but Romeo tells Friar Laurence that he cannot understand the love he has for Juliet.  Then, the friar counters,

Wilt thou slay thyself?/And slay thy lady too that lives in thee/By doing damned hate upon thyself? (III,iii,114-116)

He points out to Romeo that Tybalt--an impediment to his marrying Juliet--is now gone; the law that was death has been changed to mere exile; he can go to Juliet and comfort her, for she has "cried" on Romeo and is worried for him.  Afterwards, he can go to Mantua where he can remain until the friar can make public the couple's marriage, "reconcile" Romeo and Juliet's friends, and beg pardon from the Prince.  Then, Romeo will be called back with

twenty hundred thousand times more joy/Than thou went'st forth in lamentation. (III,iii,13--14)

Romeo agrees to this purposal, saying, "Do so, and bid my swee prepare to chide" (III,iii,147).  Then, the Nurse gives Romeo a ring from Juliet, a sign of her devotion.  He is "revived by this" (III,iii,150), and again has hope:

But that a joy past joy calls out on me/It were a grief so brief to part with thee./Farewell. (III,iii,160)

These scenes in the play are again more evidence of Romeo's mercurial nature and impetuous actions; they suggest how easily he can change in temper, a foreshadowing of possible repercussions.