Diplomacy is so complex, my friend, and trying to understand it , I myself get confused.
But I think I understand why the Chinese side are being overly critical. Japan was invited by India to invest in its Northeast States + Union Territories. Japanese companies will have the opportunity to help the development of the northeast specially to build roads, and aid agriculture, forestry and water supply and sewerage in these states. What is also important to duly note is that the Indian Government has actively discouraged and / or rejected Chinese planned investments in the Northeast of India.
The part you are talking about is poor as hell, investing there will yield next to nothing.
I doubt Japan will be that desperate to invest its money, China has those same areas, and we have to force our state ran companies to waste, I mean "invest" there.
So it is understandable why our Chinese colleagues would be overly sensitive of this 'preferential treatment' the Government of India has on Japan. China's reaction to this -- is being projected to Japan , as I'm sure Beijing does not want to (aggressively that is) over-criticize India due to the precarious relationship they have regarding border areas.
The thing about money is there's no shortage of people wanting it, if India is really pragmatic, they will let us invest, if not, it wouldn't be the right time toe invest, and we will wait until they think it over and see how it goes.
Ethiopia, and a few other African countries as well as ASEAN are nice investment countries.
Who knows, maybe ignoring investment due to "security" concerns is not a big deal and can still lead to success.
I understand your reaction. But, friend, Japan IS NOT going to be building anything in disputed Arunachal Pradesh. I reiterate, there will be ZERO (0%) Japanese development in that disputed territory. JICA will be focused on developing roads, sewage systems, manufacturing and agricultural projects in areas that are much further to the south. Areas include Meghalaya, Nagaland, Assam, and Manipur.
Please, friend, don't be too overly-sensitive. Okay?
I don't think it matters, I'm sure Xi can't be like, sorry your son died, but, if you look close we got this little piece of shit hole that's worth nothing, and kept this other piece of hole that nobody wants.
Build whatever, when the time is right, we will solve it, by just keeping what we have, the "west" is neglected for a reason.
As i said earlier. Arunachal Pradesh is not part of JICA's investment plan. Don't make a mountain of a mole-hill -- in your irritation, friend.
I say invest, invest all you want, but I'm not sure how profitable it is.