valid_input = False

while valid_input == False:
    phrase = input("Hello Dave.... are your feeling happy or sad?")
    if phrase == "happy":
        print("that's fantastic Dave.... If that changes let me know")
        valid_input = True
    elif phrase == "sad":
        print("So sorry to hear that.... I recommend going out with a mate and having a beer!")
        valid_input = True
    else:
        print("I said happy or sad?!")