Because she is having fun being near Harima of course.
Originally Posted by Swampstorm
And BTW - she knows only half of situation - she knows that Eri loves Harima. But she has no idea about the second part - and this second part is exactly the one which makes situation explosive. She doesn't know that Harima loves her. This lack of knowladge makes impossible the correct analysis of situation.
Motives of Eri back then were far from just charity onse. As far as I remember she wanted to tease Tenma, to show her that she is superior - she can freely talk with boys when Tenma can't. Tenma got angry and had her revenge.
Let's take a look at Ep.4, S1, as a parallel case. Eri sees Tenma being indecisive about Karasuma, and so she sits next to him to incite Tenma into taking action - putting herself between Tenma and Karasuma in the process. In response, Tenma became furious and got revenge by making Eri carsick. Clearly, in Tenma's mind, this principle holds as well.
She didn't take any action till nowdays only because she felt superior to Tenma and have never seen her as rival. Eri began to realize Harima's real feeling only in latest chapters, earlier she has no idea about what this feelings are.
Now, if Tenma became so angry over a single incident, imagine how angry Eri must be, after seeing the same thing happen, day in and day out. Yet, Eri never took action, all this time.
Well, it is the least harmful to both way, I agree.
Even now, the only thing that she's done is to avoid Tenma - because when a friend is constantly hurting you, the best thing to do is to stay away, rather than letting yourself be hurt. In my mind, that's more than fair
But: Eri knows that Tenma has no idea about Harima's love to her. And she does understand that Tenma is not hurting her intentedly. What really hurts Eri is feeling inferior to Tenma. Her pride is hurting Eri. Not Tenma.
If Eri would be able to suppress her pride and Tenma - to avoid Harima then will be able to reconcile. But the Harima will get hurt - "Why is Tenma-chan ignoring me?!"
I do feel sorry that Tenma is sad, but it's high time that she found out that she was being inconsiderate to her friend. It's the only way that things could ever change.
But yet, Eri is wrighting "Do what you want" and going to date instead of reconsiling with her friend. That means that man is more important to Eri than friend - which goes against Eris's words, which were said to Mikoto: "Guys over friends...you are the worst!"
Eri isn't happy about the situation either - Ch. 208 still shows her upset over her conflict with Tenma, and by Ch.209, she's so distraught that she's started to misplace all sorts of important things. Tenma isn't the only one suffering here - but until Tenma resolves the situation, nothing can be done.