When to delight and when not to delight?

I am going to assume you meant "chanda" when you say "delight" in your question.

From my best recollection, delight (chanda) in Iddhipada 4 should be cultivated. Chanda is a very interesting word Buddha used because he used in both contexts of good and bad. Chanda in 5 sensual cords is definitely not a good thing.

Also in meditation states, tho Buddha did not say directly to take delight in such, he said in many suttas that monks should cultivate them. Also for Noble (ariyan) followers, those who take delight, accumulate, stay in, expert in, and maintain the ability (of such meditation) in time of death, they will reappear in Brahma's world upon the break up of body and after that will become non-returners. So I put it together that delight in meditation is not necessary a bad thing. Might even be good.

• delight in iddhipada 4 is a good thing.
• delight in 5 sensual cords, 5 hindrances is a bad thing.
• delight in meditation state. (my own conclusion- good thing)

I am only aware of one sutta, namely, MN 1, where the bhikkhus did not delight in the Buddha's words, as follows:

Na te bhikkhū bhagavato bhāsitaṃ abhinandunti

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na (negative particle) not; no; nor; neither

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abhinandati rejoices at, welcomes; approves of; is pleased at.

Bhikkhu Bodhi comments:

The bhikkhus did not delight in the Buddha's words; apparently because the discourse probed too deeply into the tender regions of their own conceit, and perhaps their residual brahmanic views.

Bhikkhu Thanissaro comments:

The Buddha taught that clinging to views is one of the four forms of clinging that tie the mind to the processes of suffering. He thus recommended that his followers relinquish their clinging, not only to views in their full-blown form as specific positions, but also in their rudimentary form as the categories & relationships that the mind reads into experience. This is a point he makes in the following discourse, which is apparently his response to a particular school of Brahmanical thought that was developing in his time — the Samkhya, or classification school.

This school had its beginnings in the thought of Uddalaka, a ninth-century B.C. philosopher who posited a "root": an abstract principle out of which all things emanated and which was immanent in all things. Philosophers who carried on this line of thinking offered a variety of theories, based on logic and meditative experience, about the nature of the ultimate root and about the hierarchy of the emanation. Many of their theories were recorded in the Upanishads and eventually developed into the classical Samkhya system around the time of the Buddha.

Although the present discourse says nothing about the background of the monks listening to it, the Commentary states that before their ordination they were brahmans, and that even after their ordination they continued to interpret the Buddha's teachings in light of their previous training, which may well have been proto-Samkhya. If this is so, then the Buddha's opening lines — "I will teach you the sequence of the root of all phenomena" — would have them prepared to hear his contribution to their line of thinking. And, in fact, the list of topics he covers reads like a Buddhist Samkhya. Paralleling the classical Samkhya, it contains 24 items, begins with the physical world (here, the four physical properties), and leads back through ever more refined & inclusive levels of being & experience, culminating with the ultimate Buddhist concept: Unbinding (nibbana). In the pattern of Samkhya thought, Unbinding would thus be the ultimate "root" or ground of being immanent in all things and out of which they all emanate.

However, instead of following this pattern of thinking, the Buddha attacks it at its very root: the notion of a principle in the abstract, the "in" (immanence) & "out of" (emanation) superimposed on experience. Only an uninstructed, run of the mill person, he says, would read experience in this way. In contrast, a person in training should look for a different kind of "root" — the root of suffering experienced in the present — and find it in the act of delight. Developing dispassion for that delight, the trainee can then comprehend the process of coming-into-being for what it is, drop all participation in it, and thus achieve true Awakening.

If the listeners present at this discourse were indeed interested in fitting Buddhist teachings into a Samkhyan mold, then it's small wonder that they were displeased — one of the few places where we read of a negative reaction to the Buddha's words. They had hoped to hear his contribution to their project, but instead they hear their whole pattern of thinking & theorizing attacked as ignorant & ill-informed. The Commentary tells us, though, they were later able to overcome their displeasure and eventually attain Awakening on listening to the discourse reported in AN 3.123.

Although at present we rarely think in the same terms as the Samkhya philosophers, there has long been — and still is — a common tendency to create a "Buddhist" metaphysics in which the experience of emptiness, the Unconditioned, the Dharma-body, Buddha-nature, rigpa, etc., is said to function as the ground of being from which the "All" — the entirety of our sensory & mental experience — is said to spring and to which we return when we meditate. Some people think that these theories are the inventions of scholars without any direct meditative experience, but actually they have most often originated among meditators, who label (or in the words of the discourse, "perceive") a particular meditative experience as the ultimate goal, identify with it in a subtle way (as when we are told that "we are the knowing"), and then view that level of experience as the ground of being out of which all other experience comes.

Any teaching that follows these lines would be subject to the same criticism that the Buddha directed against the monks who first heard this discourse.

Translator's Introduction