Please provide at least one example of assonance in act 2 and explain why it isRomeo and Juliet by William Shakespeare

Assonance is the repetition of similar vowel sounds, usually close together in a group of words.  Familiar example are "free and easy,"  Mad as a hatter," and so on.  Assonance is a poetic device, as is most sound repetition, that is used to please the ear and to emphasize certain sounds.

In Shakespeare's Romeo and Juliet, an example of assonance is in the following lines of Juliet as the /o/ is emphasized:

And yet I wish but for the thing I have.

My bounty is as boundless as the sea,

My love as deep; the more I give to thee,

The more I have, for both are infinite.

I hear some noise within.  Dear love, adieu! (II,ii,132-136)

Likewise, the character of Romeo employs the poetic device of assonance in his life earlier to Juliet:

With love's light wings did I o'erperch these walls,

For stony limits cannot hold love out.

And what love can do, that dares love attempt,

Therefore thy kinsmen are no let to me (III,ii,66-69)

In fact, there is much repetition of the /o/ in Act II.  For one thing, the word love is repeatedly throughout the scene.