Why does Romeo keep secret from his friends the reason for his dramatic change in mood in Romeo and Juliet?

After the night in which Mercutio and the others search
for Romeo, who has scaled the orchard walls of the Capulets in order to see with love
dazzled eyes she who "doth teach the torches to burn bright,"  Romeo visits Friar
Laurence.  At the priest's cell, Romeo asks the friar to perform the marriage rites
for Juliet and him.  Amazed, Friar Laurence exclaims,

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Holy Saint Frances, what a change is
here!

Is Rosaline, that thou didst love so
dear,

So soon forsaken?  Young men's love then
lies

Not truly in their hearts, but in their
eyes.

Jesu Maria, what a deal of
brine

Hath washed thy sallow cheeks for
Rosaline!....

The sun not yet thy sighs from heaven
clears,

Thy old groans ring yet in mine ancient
ears.

Lo, here upon thy cheek the stain doth
sit

Of an old tear that is not washed off
yet....

Thou and these woes were all for
Rosaline.

And art thou changed? 
(2.3.68-77)

Since the priest
has chided him and since the Capulets are the sworn enemies of the Montagues, Romeo may
be more than a little reluctant to reveal his infatuation and increasing passion for
Juliet.  Certainly, he does not want to broach the subject when he meets up with
Mercutio, his cousin, who is engaged in a heated dialogue with Tybalt in the streets of
Verona.  And, it is possible that Romeo considers the parting words of the priest,
"Wisely and slow.  They stumble that run fast" (2.3.97)