A real nice guy is a guy who is just NICE. Kind, caring and understanding but without expecting a reward for it.
A self declared nice guy however is usually a guy who will be nice to you and expect some sort of romantic or sexual reward.
The self declared nice guys usually are the ones who complain about being friendzoned.
Real Nice Guy: Helps you with your problems and provides an amazing friendship without getting angy if you don't want to have sex with him, or date him, afterwards.
Self Declared Nice Guy: Helps you with your problems and provides an amazing friendship, then gets angry when you don't jump in bed with him for it.
Girls are not machines that you put kindness coins into until sex falls out.
Being nice to her doesn't make her obligated to develop sexual and/or romantic interest in you.
NOT LIKING YOU DESPITE YOUR KINDNESS DOES NOT MEAN SHE ONLY LIKES DOUCHEBAGS.