When you are blessed, you are to be a blessing to others. Your success and happiness ought to bring success and happiness to others.
This is not stated as a guilt-tripping obligation, but simply as a proper ordering of things and a preventative against goodness going bad. Genuine blessing begets blessing. Just as offspring beget offspring, goodness begets goodness.
When you are blessed and you fail to let it spill out onto others and multiply, you are turning the blessing inward. What would be a joy and strength to you turns into a poison. Your cup is meant to overflow, not just to grow ever larger or be pressure-sealed. It will rupture or corrode your heart if you try to keep it in you, instead of letting it flow through you. A pond grows stagnant while a stream remains fresh.
Good comes into the world through good. If good happens to you, you can grow the goodness by letting it overflow to others. Bless with your blessings.