@Akshay_horny - good summary, but it goes a lil further back. The boundaries between the two nations were made by British, post WW2, but never were formally agreed upon. (it is the same case of Islands in the South China Sea) And its madness since then. As in all battles, high places, glaciers, passes are important and the Himalayas are full of those.
Moreover, India's political asylum to Dalai Lama also never went well with China. DL has spoken and raised voice against the occupation of Tibet. And we know how speaking on that lines go.
To add to that China favours and supports Pakistan, Sri Lanka, and lately Nepal - via various means, that too creates loads of problem. The India-Pak relationship is world-famous, India - Sri Lanka though on non-confrontational status but China has influenced Sri Lanka and got a massive port.
China is looking forward to creating a "String of Pearls", so that they can counter the naval presence of western nations, as well as assert dominance in South East Asia Sea and the Indian Ocean. Sri Lanka, already on board, Mynamar, Bangladesh, Pakistan, Maldives and Somalia also on board. Port work is on most of these nations. This covers one of the worlds busiest maritime zone.
If you want to read up on the planned String of Pearls
https://en.wikipedia.org/wiki/String_of_Pearls_(Indian_Ocean)