I think you need more and more interactions per Sweet Heart as you
send out more Valentine cards.
Have noticed at least need double amount of interactions for a
single Sweet Heart as I sent out around 8-9 cards.
As I can see while I interact, Sweet Hearts are completely obtained
randomly
When I click yesterday, sometimes 1,000 interactions only give me 5
Sweet Hearts, but 2,500 interactions give me 15 Sweet Hearts
So, basically, random