Juliet thinks that Romeo may be playing games with her, and becomes wary of this. Why does she think like this?

Romeo is pretty aggressive. He is already talking marriage before they have really had a conversation. They really do not know each other but Romeo is comparing elevating Juliet to a goddess.

But soft! What light through yonder window breaks?
It is the east, and Juliet is the sun.
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief,
That thou, her maid, art far more fair than she.
Be not her maid since she is envious.
Her vestal livery is but sick and green,
And none but fools do wear it. act 2 sc 2