Why Are My Sims Such Nimrods?

I don't have so much experience with teens, but i should soon as my sim child is about to age up. But as I started with 3 sisters, young adults and i created a plan how I want their stories to be, I was ready to marry off one of the sisters and move her out. So, as for me it is more realistic a man proposing a woman, I went through the manage world and was excited her boyfriend to propose her. They had a wonderful date and everything worked well. They were best friends too. And then this very romantic moment arrived...he got on his knees and asked her to be by his side forever, but she rejected him!! Omg i was not expecting it at all... so i tried next sim day again and once again she rejected him. She has no traits to do so, but she sure has some character to surprise me. So i thought fine, she doesn't want to get married to that man. They broke up and she started to date another man. All was good until this man proposed her. She rejected again...and again...and again.....So at the end I made her propose a vampire as a punishment ?. This vampire didnt reject her... so now she will get married to a vampire.