Where is your private key located? By default, ssh looks for a key in ~/.ssh/id_rsa. It sounds like your key is stored somewhere else, so that’s why you’re unable to connect without explicitly specifying the path to your key.

You can either move/copy your SSH key to that path, use ssh-agent, or modify the config file ~/.ssh/config to include the existing path:

Host *
    IdentityFile /path/to/private_key