I have changed service file like this. Now ssh keys are added on login and no need to enter passphrase every time.
[Unit]
Description=SSH agent (ssh-agent)
[Service]
Type=simple
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
Environment=DISPLAY=:0
ExecStart=ssh-agent -D -a $SSH_AUTH_SOCK
ExecStartPost=/bin/sleep 3
ExecStartPost=/bin/sh -c ‘/usr/bin/ssh-add /home/user/.ssh/id_rsa’
ExecStop=kill -15 $MAINPID
[Install]
WantedBy=default.target