why does Morocco doubt his worth to deserve portia

Morocco is trying to decipher the cryptic messages of the caskets. He sees the silver one which says, "Who chooseth me shall get as much as he deserves.” Morocco thinks he deserves a lot but perhaps not enough for the beautiful Portia,

As much as he deserves!”—pause there, Morocco,

And weigh thy value with an even hand.

If thou beest rated by thy estimation,

Thou dost deserve enough, and yet enough

May not extend so far as to the lady.