Try the following:
'''1. Delete Preferred Network(s)'''
o System Preferences > Network > AirPort > Advanced > AirPort tab
o Under "Preferred Networks," delete the network(s) you regularly use from the list.
'''2. Delete AirPort Keychain Entries'''
o Launch the "Keychain Access" application located in Applications/Utilties.
o Click on the "Kind" filter at the top, and look for any "AirPort network password" entries...and delete them.
o Restart, or log out then back in.
'''3. Add Preferred Network(s)'''
o System Preferences > Network > AirPort > Advanced > AirPort tab
o Add the preferred network(s) using the "+" button.
o Restart or log out then back in.