Seeking approval

Hello, my teen daughter has this thing where she is overly generous to the point that she gets abused. She's always been very generous since she was little. Always wanted to donate things and give people her possessions. However, now that she's older, I see this play out over and over where her "friends" just take advantage of her. When I point it out she gets mad and says that I don't understand her, that her friends also do things for her (very minimal compared to what she does for them), etc. She's given a lot of her lunch and money and other things away, and I don't know how to stop this without squashing her generous spirit. I feel like maybe she does this to be liked by her peers although she's denied that. Help!