If you're not humble, you believe you deserve to be happy. It is highly unlikely you will believe you are exactly as happy as you deserve, so sooner or later it will occur to you that you aren't as happy as you deserve. (If you believe you're happier than you deserve, that's humility.) If you think you aren't as happy as you deserve, you'll feel cheated out of something, and that will spoil whatever happiness you do have, and you'll be miserable.
Someone who's humble, on the other hand, doesn't think he especially deserves to be happy, and is therefore surprised and grateful when he is and not overly disappointed if he isn't.
So try to be humble. If you can.