“heartstrings” is a term that comes from the strings on an instrument—you pull on the strings to make a sound. to “pull someone’s heartstrings” means that someone has had their feelings of affection or compassion (the strings) pulled on by something someone else did/said. basically, the MC was so pitiful and sweet it made the ML feel concerned for him and feelings of affection.
more examples:
you see a baby start to tear up and pout and it pulls on your heartstrings.
a puppy howls really cutely and it pulls on your heartstrings.
a toddler tries to count all the way to 100 but forgets the numbers and it pulls on your heartstrings.
"All those ways he's pulled on my heartstrings". Can someone please explain what he meant by that? English is not my native language and I struggle with such expressions :///