Why do Mercutio and Benvolio think that Romeo avoids them after the party?

In Shakespeare's Romeo and Juliet,
Benvolio and Mercutio think Romeo has gone off to be alone or sulk, because Rosaline
does not love him. 

They look for him and, at first,
Mercutio says that Romeo "hath stolen him home to bed."  He says he's gone home to
bed.

Benvolio, however, points out that he saw Romeo leap
over an orchard wall:

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He ran this way, and leapt this orchard
wall.

And Mercutio assumes he
has gone to sulk.  He makes fun of Romeo, pretending to conjure him up in the name of
Rosaline:

I
conjure thee by Rosaline's bright eyes,

By her high
forehead and her scarlet lip.

By her fine foot, straight
leg, and quivering thigh,

And the demesnes that there
adjacent lie,

That in thy likeness thou appear to
us.

And Benvolio, telling
Mercutio that it's time to go,  concludes that Romeo

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...hath hid himself among these
trees

To be consorted with the humorous
night.

Blind is his love, and best befits the
dark.

Of course, Romeo is now
in love with Juliet, rather than with Rosaline.