I do it because I don't believe it from my perspective. That I'm not good enough.

Well, not just rude it can deflate people having their genuine compliments rejected over and over. Unheard.

They're not used to and they don't know how to take a compliment and how to react to them. I'm one of those people sometimes. I feel uncomfortable and ashamed when I'm beinf complimented most of times

Because to accept the compliment would be to change their negative self-perception, and one person saying something isn't cathartic enough for them to do that. I don't think it's rude.

If it is a genuine good faith compliment, it's prolly insecurity. I don't think it's rude. I think people should give compliments because they believe what they're saying, not because they need the other person to accept the compliment

I think this has nothing to do with being very modest. It seems to me that it is rejecting a chance of getting on a more personal level it is a defense against the personal touch. When you compliment a person and the person accepts it you both share a bit of liking, respect, extra friendliness. Some people hate that.

Rudeness to me is mostly less about what you do and more about how you do it, as for why...lots of people are walking around with trauma and shit, you walk up to someone you don't know and start commenting on them (or it could be someone you do know, just not well enough), you never know what nerve you're going to hit and you should be ready for that and maybe a little understanding instead of being appalled that someone feels a certain kind of way about what you said, whatever your intent

When you lack enough self-confidence, you will tend to believe compliments are lies. Also what Shannon said about flirting, those can be somewhat shallow if you're doing it out of horniness.

I think it's only rude when the rules above don't apply.