[Fizzlemizz]

The basics would be something like:

local maxNotifications = 3
local f = CreateFrame("Frame")
f:RegisterEvent("UNIT_HAPPINESS")
f.happiness = 0
f.happinessCount = 0
f:SetScript("OnEvent", function(self, event, ...)
    local unit = ...
        if unit ~= "pet" then return
    end 
    local happiness, damagePercentage, loyaltyRate = GetPetHappiness()
    if self.happinessCount > maxNotifications and self.happiness == happiness then
        return
    end
    if happiness ~= self.happiness then
        self.happinessCount = 0
    end
    self.happinessCount = self.happinessCount + 1
    self.happiness = happiness
    if happiness == 1 then
        UIErrorsFrame:AddMessage("FEED YOUR PET ALREADY!!!", 1.0, 0, 0, 1.0)
    elseif happiness == 2 then
        UIErrorsFrame:AddMessage("Your pet is a little miffed with you!", 1.0, 1.0, 0, 1.0)
    end
end)

UNIT_HAPPINESS is a rolling event so as is, this only shows a message 3 times when the happiness level changes.