Reason # 1.
Nepal have ignored China for last 200+ years while Nepal have open border with India.
This have created unbalance. I think Nepal should open border with China as well.
Let in Chinese investors, business entrepreneurs venture into Nepali market.
The same way Indian have access to Nepali market.

Reason #2.
China to be the most powerful nation witthin 5-10 years economically and in militarily.
Nepal gains more by maintaining stronger relationship with China than by closing the border.

Reason #3.
We could tap in the railway network  from Lhasa, Tibet to Kathmandu.
Nepal-China could build more roads linking from Kathmandu to Tibet and other parts of China.
This we can have only by maintaining stronger ties with Chinese government.