I feel lightly offended when insecure people turn down my compliments

I can't say I'm not insecure in my own right, but when I try to boost my friend's confidence by complimenting their art or something, instead of getting a simple "thanks" or whatever I usually get something like "thanks, its not that good though," or "I didn't put that much effort into it." I know this is a default response for people with lower self-worth, and I don't blame them, nor do I try to correct their responses. But it always feels so unintentionally rude, like they're turning down my compliment or something. I can't say I haven't done this myself, and I really am trying to work on it, because I know if it bugs me that much, I shouldn't be a hypocrite. But my friend will take it to such unfortunate levels that it almost feels unintentionally offensive, hearing them say that they're somehow the ugliest person on earth, when the reasons they have for insulting themselves would still look fine on other people, or heck, maybe even I have those traits, and that's why it kind of hurts to hear. I hope this made sense. I just wanted to know if it was wrong for me to feel internally hurt every time my compliments feel like they're just being outright rejected sometimes, even if not on purpose.