For some reason my Nokia Lumia 920 and my MacBook don’t get along where internet sharing over wifi is concerned. The MacBook continually drops the connection with the cryptic message “kernel[0]: AirPort: Link Down on en0. Reason 7 (Frame received from nonassociated STA).” After much searching and failing to find a cure, I fashioned a good old-fashioned command-line band-aid:

while true;
    do if ping -W2 -c2 $(route -n get default |grep gateway | awk '{print $2}') > /dev/null;
    then echo "still up";
    else echo "reconnect";
    networksetup -setairportnetwork en0 MY_NETWORK_NAME MY_NETWORK_PASSWORD; 
    fi;
done