Which summoning spells conjure willing creatures, and which unwilling?

Question

Certain summoning spells seem to force unwilling creatures to do your bidding - examples include the D&D 5e spells Conjure Fey and Conjure Elemental, where the summoned creature becomes hostile if you lose concentration. Others, such as Planar Ally, explicitly provide the help of a willing creature (even if they ask for payment in return).

But many spells, as described in the Player's Handbook, do not indicate whether the summoned creatures are willing or unwilling. For instance, Conjure Minor Elementals states, "The summoned creatures are friendly to you and your companions", and they will never become hostile to the party. However, that exact wording is used for Conjure Fey and Conjure Elemental too, applying before you lose concentration. This might mean that the "minor" summoning spells summon only willing creatures; or that the creatures are too weak to break your mental domination.

Are there any official explanations (i.e. fluff or flavour text) of which summoning spells conjure willing creatures, and which unwilling? This could be game materials like a DM's Guide (I already have the Player's Handbook), but also official tie-in novels, adventure modules, or magazine articles. Interesting examples might include a depiction of a Good-aligned character refusing to summon unwilling creatures, or showing such a spell from the summoned creature's point of view. D&D 5th Edition is preferred.

I have already found: an unsourced statement that Planars do not like being summoned (Planescape, unknown edition); in 3.5e, Gate specifies that the creature might be willing or unwilling; 3.5e also features the spells (Lesser/Greater) Planar Ally that explicitly call a willing being, and Summon Nature's Ally spells that suggest willing creatures by their name.

Answer 1

Most creatures are summoned unwillingly, or are otherwise unhappy about being summoned.

The Monster Manual is rife with examples of creatures that hate being summoned, under almost any circumstances. For example:

Devils: MM (67):

However it is summoned, a devil brought to the Material Plane typically resents being pressed into service. However, the devil seizes every opportunity to corrupt its summoner so that the summoner's soul ends up in the Nine Hells. Only imps are truly content to be summoned, and they easily commit to serving a summoner as a familiar, but they still do their utmost to corrupt those who summon them.

Yugoloths (a specific circumstance) (MM 311):

A yugoloth summoned using its true name, as inscribed in the Books of Keeping, is forced to serve its summoner obediently. The yugoloth hates being controlled in this manner and isn't shy about making its displeasure known.

Elementals (MM 123):

Certain spells and magic items can conjure an elemental, summoning it from the Inner Planes to the Material Plane. Elementals instinctively resent being pulled from their native planes and bound into service. A creature that summons an elemental must assert force of will to control it.

Demons (MM 53):

A mortal who learns a demon's true name can use powerful summoning magic to call the demon from the Abyss and exercise some measure of control over it. However, most demons brought to the Material Plane in this manner do everything in their power to wreak havoc or sow discord and strife.

(MM 51):

If a single mistake is made, a demon that breaks free shows no mercy as it makes its summoner the first victim of its wrath.

One exception is a Glabrezu (MM 53):

A glabrezu takes great pleasure in destroying mortals through temptation, and these creatures are among the few demons to offer their service to creatures foolish enough to summon them.

The Gate spell is explicit about simply pulling any creature away without notice:

When you cast this spell, you can speak the name of a specific creature (a pseudonym, title, or nickname doesn't work). If that creature is on a plane other than the one you are on, the portal opens in the named creature's immediate vicinity and draws the creature through it to the nearest unoccupied space on your side of the portal. You gain no Special power over the creature, and it is free to act as the DM deems appropriate. It might leave, Attack you, or help you.

The Conjure Fey spell implies that the fey resents being summoned, since it attacks you if you lose concentration (much like a demon):

If your Concentration is broken, the fey creature doesn't disappear. Instead, you lose control of the fey creature, it becomes hostile toward you and your companions, and it might Attack.

Confusingly, the Conjure Woodland Beings spell does not have that clause. Indeed, you could summon the same creature with either spell, and it will only attack you when you lose concentration only if you used the higher level version.

The only case that might allow for some free will is Conjure Celestial, because it has the freedom to only obey commands that follow its alignment.

It obeys any verbal commands that you issue to it (no action required by you), as long as they don't violate its alignment.