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.