There are several historical reasons why "it's complicated". The North Korean state, or DPRK, was basically created by the Soviet Union with Kim Il-Sung as their puppet in 1945. For the first few years until some time between after the Korean war (1953), he was very much in their pocket, and the DPRK mostly tried to be a model communist state. The war saw massive military aid from China, and switching from purely Soviet to mixed Soviet-Chinese sponsorship was done in part by the development of Juche (self-sufficiency/nationalism with a personality cult), a local and definitely non-Soviet, not-really-communist ideology adopted by the DPRK in 1956. In 1961, the Mutual Aid and Cooperation Friendship Treaty was signed between the DPRK and China, with a gradual shift in the primacy of international relations to that country.

The final nail in the coffin was the establishment of relations between the Soviet Union and South Korea in 1990, and it's collapse the year after. DPRK now found itself with only one possible ally - a nominally communist country with which it shares a border and some fragments of ideology - state control, Confucian and shamanist practices, strongman-type leaders with a personality cult. It was also useful and expedient for China to have a buffer state against US-controlled South Korea, as well as an (at the time) not insignificant trading partner.

In the post-Soviet era, then, China has been the DPRKs biggest trading partner, protector in international relations, and a large source of international aid. In return, the DPRK has given back the gift of its continued existence, rather than collapsing with millions of refugees streaming into China over the Yalu river.

Sources: Bradley K Martin "Under the Loving Care of the Fatherly Leader-North Korea and the Kim Dynasty"
Jang Jin-Sung "Dear Leader"
Barbara Demick "Nothing to Envy"