I think Inara is able to understand Rose so well because they are in similar situations. Travelling with a man they want to admit their feelings to but can't because said man really isn't up to admitting his feelings either. In Inara's case, she decides to cut ties before it can reach the level of devotion that Rose has for the Doctor. That's the contrast I enjoy between Inara and Rose. Inara is the companion who left while Rose is the companion who stayed. In the end, their roles end up reversed.

Gosh, those two look so great together and I'm not just saying that because I made the icon. ;-) I want more icon space just so I can have more Ten/Inara icons.