I once had a "nice guy" who thought he could get me to change my mind into liking him by being nice to me despite telling him repeatedly that he was not my type. Also, he would constantly dead name me with "cute" nicknames that played off my dead name. Like he knew why I changed by name from the one I was given by parents who verbally, psychologically, & emotionally abused me but thought he could just fix it by giving me nicknames instead of using my preferred name which he only did occasionally. Once he started going into transphobic & homophobic stuff, I blocked him & stopped interacting with him entirely. Apologies for my little ramble there, but yeah, nice doesn't mean you're entitled to anything beyond a polite response of thank you
Hal's character in Megamind vs Megamind is a perfect example of "nice guy syndrome". Hal does things for Roxanne and expects her to love him in return while Megamind loves Roxanne and is purely happy just spending time with her, even if he gets nothing in return. He becomes a better person because of Roxanne and still says he understands if she's not interested.
Being nice is the bare minimum, it doesn't make you entitled to anything. Also nice people don't have to say they're nice, people just immediately know
I knew a "nice guy" who I had to block because he kept coming onto me aggressively despite me being in a relationship and ace. The worst part is that I met him as a minor and he was an adult.