My view is that she didn't seem to have anyone else in her life and Riley did so much for her that she felt really grateful for. Riley on the other hand, threw away her firefly pendant and claimed she didnt know why she came back and risked her life just to hang with Ellie in the mall and do what she did for her. I think Riley, being the older, absolutely had some feelings for Ellie and Ellie felt something too although how strong they are are still not exactly clear.
One interesting train of thought is Spring. This DLC shows that Ellie, by saving Joel, reflects back upon losing Riley. So by the time of Spring when Ellie is all silent and emotional... could it be because she is thinking of Riley? She even mentions Riley towards the end of the main game in Spring. Infact it all adds up to me.
Other then that... damn I loved it. It was somewhat short but hell it is a DLC, it just means that I never wanted it to end haha. It was a bit slow for a few moments but given the context, I can see the purpose for it. Incredible DLC that further reinforces Ellie as the greatest gaming character. 9/10.
