Why do the Chinese consider India as their enemy?

In an article on the 60th anniversary of the People's Republic, Times Online makes a remark. 

Curiously, the enemy most often spoken of is India. The censors permit alarmingly frank discussion on the internet of the merits of a war against India to secure the Tibetan plateau. 

However, the article falls silent on why the Chinese considers India as its enemy number 1 and gives only a tenuous and indirect reason by quoting an unnamed contributor. 

"Help the Maoists take over power in India to pay them back for hosting the Dalai Lama". 

What do you make of it? Is this the reason?