Hey reddit

I'm turning 21 this year and think I'd like to celebrate this year since it symbolizes the start of my adult life. My plan is to book an Airbnb or a hotel and invite my close friends. But since it does cost some $$, I asked my friends to split the bill. Most of them were chill about it, but there is that one friend turned me disappointing. She and I had a lot of time spend together since elementary, we are still in touch even after she went to an out of state university. I don't understand why she is mad about me asking her to split it and help me some bit.