Because faith is the strong believe in God and when we ask Jesus we are sure that He will give us what we want becuase He is a mercy and loving God and He gives us all we need.   And we also accept His will even if we don't get what we ask for; maybe it is not good for us. God know what is better for every person.

If we have faith like a mustard seed and we trust the living God, we are going to live a happy life and although we were sick our faith in God would make us feel better.