What does the dialog related to Nanami's quote in the last episode of Bokura Ga Ita mean?
The answer follows when Yano entered the train:

 &quot;Because I met Takahashi. I lived to meet Takahashi.&quot;

 In the end, the positive side is that at last Yano admitted he doesn't feel lonely anymore because he has Nanami.

Here is the original dialog in Japanese

七美　「どこにいてもどんなにさみしくてもつらくても
　　　　ここで七美が想ってることを忘れないでね。
　　　　矢野は絶対、ひとりじゃないから」
矢野　「プラマイゼロ？」
七美　「え？」
七美　『矢野のすごくすごく好きな人はもういないのかもしれないけど
　　　　それはさみしいことなのかもしれないけど、
　　　　でも矢野のことをすっごくすっごく好きな人間がいるってことはそれって
　　　　プラマイゼロじゃないかなあ。だからひとりだと思わないでね』
矢野　「そうじゃなかった」
矢野　「ゼロじゃなくてプラマイプラスだったよ」
矢野　『高橋と出遭ってから、俺は高橋と出遭うために生きてきた』
Dialog source: Happy☆Lucky's blog entry (Japanese)

In the original Japanese scene, Nanami used the term &quot;plus-minus zero&quot;, in which Yano replied with &quot;plus-minus plus&quot;.
Also, the main point of Nanami's quote is more about

 loneliness, deeper than just &quot;someone who likes/is liked by another&quot;. Nanami felt that Yano was feeling lonely after his previous girlfriend had left him, but since Nanami liked Yano, he shouldn't be feeling lonely anymore, hence &quot;it all equals out&quot;. But Yano couldn't move on despite that.

However, after having gone through all of this, Yano finally realized that he was wrong, hence &quot;it ended up [...] on the positive side&quot;.