Konfrontasi (or Confrontation, 1963–1966) was Indonesia’s response to the formation of the Federation of Malaysia, arising from the British decolonisation process in Southeast Asia. Konfrontasi involved armed incursions, bomb attacks and other subversive acts aimed at destabilising the states that were to be included in the Federation, namely, Singapore, Malaya, Sarawak, Brunei and North Borneo (now known as Sabah).
When the concept of Malaysia was first mooted publicly by Malayan Prime Minister Tunku Abdul Rahman in May 1961, the Indonesian government initially did not raise any objections. It began to express opposition to the Malaysia proposal shortly after the Brunei Revolt of December 1962.
You may be satisfied with the above. Or you might dig deeper. Like what were the basis of Indonesian objection or opposition to the formation of the Federation? Further along, you read:
Indonesia criticised the Malaysia plan as a British “neo-colonialist project” and a threat to their country’s security.
Which is diplomat-speak for "because I don't like!"