Just like how the MC would do everything for the person they like but get nothing in return.
People are stupid. They value their emotions far more than the truth.
The things you do for people you love, you considerate it worth it because your emotions gives it a false sense of value. But the things that people do for you, you don't see it worth anything because you don't have your emotions to give it a false sense of value.
Emotions blinds people from the truth. Which makes them stupid. It's only when you realize that the love you give is never gonna get return that you realize how foolish you are for giving it. And it is only when you lose the love that is given to you are you able to see its worth.
People can only see the truth when it is already too late.
True man true
now i even sad-er, but that's life i guess
SPOILER:
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It's not all sadness for BFF-san! It gets worse for a bit, but later it gets better. This manga is really good at character development (and art)
i hate the best friend type. they always end up loving the MC and get nothing back but sadness and pain and make me feel sad ╥﹏╥