Happiness is a transient achievement. Life is a rollercoaster, more ups and downs than you can ever imagine and sometimes at a speed that is less than comfortable.
You sometimes only know you were happy when you look back on events, and realise how much you can take for granted when it is suddenly taken away from you. Other times you are so blissfully wrapped up in happiness it is almost visibly glowing from you and can be infectious... look on both these times with joy and remember them; they will keep you sane in the darker less happy or sad times.
Remember that this is a wonderful world, with miracles happening every day somewhere, a beautiful sunrise, or sunset; the sound of a bird singing in the early dawn heralding the spring and rebirth of warmth (well ok optimistic that's me sometime); the voice of the person you love saying good morning and I love you.
Take nothing for granted, give happiness to others where you can it might only take 5 mins of your time but will mean a lifetime of memories to others.
Above all, live for today for tomorrow we do not know where our paths may take us.