Suppose that you started out this new year, promising yourself that you'll have fun. Lots of it - uninhibited, unadulterated and guilt-free fun. Now assume that you even did (no, I'm not passing judgment on your sad lives :)) Today's the twenty sixth of February. Today, you're feeling all down and in the dumps. As happens with prolonged low periods, you tend to twist and corrupt every good memory of yours so that it suits your purpose. For instance, in a sad state, you'd tend to look at the sad parts of all those memories. Or, when you're accusing your spouse of cheating (I hope it doesn't come to it) you'll tend to look suspiciously at everything you guys have done together.
Similarly, with sad states, you'd tend to look at everything through that lens, and you'd feel that you never had fun. Now turn the situation on its head. Assume that you've had a super sad year so far, but today, for some reason, is super happy, optimistic and so on. Won't you have a positive outlook about everything in life?
I know it sounds somewhat simplistic, but do think about it - whatever we do today, does it have such strong a connect with the past or the future as we make it to have? My guess is that putting it at even the tenth fraction would be overstating it.