My 0.01 cent opinion: "Y" = you, because no one else will come to help but yourself. Cavalry is not comming, everybody is busy on their own campaigns.

I may be wrong, but this is just my opinion. Perhaps the director and the script writer design that title like that to tickle our minds and let us think a moment why "Happyness" is spelled with a "Y" and not an "I"- just a wild guess.

I love the movie, it is my inspiration and motivation in life. This movie also uplift me when ever I am down with life struggles.