You need to be clear about the difference between a password and a passphrase - if it's asking for a password then it suggests it's not finding/recognising the key and is dropping through to regular password-based authentication but if it's asking for a passphrase then it wants the phrase you entered when generating the key pair - you can set a blank phrase if you don't want to be prompted but obviously that loses one factor in security
Normally this program generates the key and asks for a file in which to store the private key. The public key is stored in a file with the same name but “.pub”
appended. The program also asks for a passphrase. The passphrase may be empty to indicate no passphrase (host keys must have an empty passphrase), or it may be a
string of arbitrary length. A passphrase is similar to a password, except it can be a phrase with a series of words, punctuation, numbers, whitespace, or any string of
characters you want. Good passphrases are 10-30 characters long, are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per
character, and provides very bad passphrases), and contain a mix of upper and lowercase letters, numbers, and non-alphanumeric characters. The passphrase can be changed
later by using the -p option.