Why do some men exploit a good woman who is willing to give him her all and supports him to the best of her ability? [19F] he’s [21M]

Why do some men exploit the kindness of a good woman who would give them everything and go out of her way to support them? I (19F) have been best friends with this guy (21M) for a year, and over time, we've developed feelings for each other. Initially, everything seemed great, and I felt a strong connection between us. However, as I began to show him more love and commitment, I noticed a change in his behavior. It feels like he’s taking my kindness for granted. He’s increasingly putting less effort into our friendship, as if he knows I’ll always be there. He struggles with mental health issues, which I completely understand, and I’ve tried my best to be supportive, but it’s becoming emotionally exhausting for me. It often seems like he doesn’t take my feelings into account, and I’m left feeling like I’m the only one investing in this relationship. He doesn’t have a job and struggles with sleep, caught in a negative cycle that’s hard to break. While I get that it’s tough for him, it’s also challenging for me to keep giving without much in return. He has admitted to having feelings for me but has stated he can’t promise anything because of his mental health issues. While I appreciate his honesty, I find it difficult not knowing the direction of our relationship. Over the past year, we’ve formed a strong bond, but now it feels stagnant. I know you can't rush someone’s mental health journey, but I had hoped my support would make a difference. I've considered cutting ties, knowing it might be the best option, but the thought of losing him, along with our friendship, is painful. I’m deeply attached, and it hurts to feel unvalued. He has mentioned that other guys would be interested in me and encouraged me not to wait for him, yet he also struggles to distance himself from me. He believes he’s too much for me to handle, but I can’t seem to let go. I’m at a loss for how to foster a healthier dynamic between us. I desire a relationship, but I also want him to recognize and appreciate everything I do for him. I’m unsure how to show him that I’m worth his effort. Many people advise me to distance myself, but I don’t want to lose him. I would appreciate any advice on how to navigate this situation or how to encourage him to value me more. Thank you in advance.