The thing about Riley in terms of Ellie and her own trust issues and character is that everything that Is Ellie would never have been possible without Riley.
Riley was the first person that Ellie loved, her first crush, her roommate, her best friend. She was also the first person to leave without saying anything, not even telling Ellie anything about it. Riley unintentionally gave her that bit of guilt and trauma that she was so easy to be left behind (which I think is a big reason the dlc/episode is titled in such a way, to symbolize not only Ellie's feelings and emotions, but also her first home that she can never go back to, of Many what ifs that could have happened if that night, or any other night before or after that, had went differently). Riley is also the person that matters to also (unintentionally) put it into her head that she doesn't belong anywhere, that she wouldn't know what it was like to have someone love her like Riley's parents had unconditionally loved her.
Joel is the first person to treat her in such a way without thought. Just within hours of meeting each other, he kills someone for her, someone that he knew and was friends with, because he was going to kill her, and he feels the same fear and anger he felt when Sarah had also been shot so carelessly by someone unwilling to listen. He drags her along if he has to to keep her alive, does what he can to make sure she's safe and warm and fed.
So, naturally, when the next person who she comes to trust again that isn't dead says he wants to leave her? Well, originally, she runs. But, here, she stays, but the reactions are the same; He's the first person to treat her like a kid, to protect her by practically ripping people apart for trying to hurt her, and he's the first person to not leave. That means everything to Ellie, it's everything she's ever wanted, to not be alone, or alone with someone she can't feel safe with. Of course she's going to be angry and upset!
But all of this makes Maria's talk with Ellie that much...More. The only people that can actually harm you aren't the raiders or soldiers that want to kill you (or Worse), but the people that you trust.
















