When we share merits with other beings, what they receive is not our good kamma because in every being’s stream of consciousness, they have a potential store of wholesome and unwholesome effects that can take place. 

When they hear and they can rejoice, that wholesome mind opens the door for their wholesome kamma to enter their stream of consciousness.

They’re not getting our good kamma; they’re allowing their good kamma to come through only if they can feel the gratitude or happiness that someone is trying to share.

It’s like you’re full and you wish another person ‘I hope that you’re not hungry’, it doesn’t make them not hungry. But if they can feed themselves – in this case feed themselves with wholesome thoughts – then, that fulfills their good kamma.