dragons are strange creatures. they are immortal, and gain knowledge at a similar rate to humans. they are undoubtedly sapient and fiercely intelligent when they live long enough to safely learn. they are fickle beasts, however, and have an inherently different set of values than humans do. greater dragons, being solitary creatures, differ quite a bit from lesser dragons, which are communal.
bargains, contracts, and pity- greater dragons
to put it bluntly, greater dragons think themselves superior to everyone else. from an early age they are able to hunt and fight for themself. they also grow quickly, reaching a suitable juvenile size within a year. they will grow for the rest of their lives, but at a much slower pace after the first year. they rarely talk to each other, let alone other species. why trust their eggs to humans?
simply put, free childcare. greater dragons, after laying their eggs, don't care much for parenting. they are high-energy beings and need to feed. like many reptiles, they lay their eggs and go. typically the eggs are numerous or tough-shelled enough so that a small number of them make it to adulthood. (not too many, though, we don't want territory disputes!)
due to their seemingly infinite lifespans, dragons do not experience the same sort of evolutionary pressure to procreate. they are deeply prideful creatures, however, and greatly value their legacy, and proof of a strong bloodline. humans, then, are advantageous. why worry about your nest being stolen or destroyed if you simply cut out the middleman and work directly with humans? greater dragons with satisfactory ability and strength of blood can easily escape any sort of human situation once they get old and big enough- so there is a guarantee any sort of confinement is temporary- and working with humans has the added benefit of whatever the humans can teach them.
dragon riders are those who are tasked with raising baby dragons to their full juvenile size- teaching them to hunt, to fight, sometimes to speak or understand language- with the understanding that they will be released someday to fend for themselves. those families who gain the reputation for bringing up well-prepared dragons may sometimes find their proteges willing to lend a claw for their prosperity in the future.
aside from those dragons "born into" a dragon rider stable, rarely some dragons make pacts, or contracts, with certain individuals, for power or renown. frequently those with the 'family dragon' are well-established noble families, or those with royal blood.
some dragons really like esteem, whether that's from their own kind, or from humans.
mutual benefit and symbiosis- lesser dragons
lesser dragons, by definition, live in groups. sometimes called clans, sometimes called packs, each group tends to raise its children together. lesser dragons are smaller than their "greater" counterparts, which makes it vastly beneficial to have strength in numbers. it's not common that lesser dragons entrust their eggs or their young to humans or other species. usually those in stables were from abandoned nests, split groups, or some sort of incident that made raising those eggs unsustainable for the clan.
lesser dragons can just as easily destroy their human captors as greater dragons, but due to their communal nature, are seen as more compliant. the bonds formed between riders and lesser dragons are said to be far stronger than those with greater dragons. it is expected to be an equal partnership.
lesser dragons can be seen in a wide variety of different contexts- from guards, to beasts of burden, to storytellers and officers of the state (the latter are only really found in giant communities)
lesser dragons aren't as common in riding community (as a practice that grew out of war, and feats of combat) but there is talk of establishing separate leagues specifically for lesser dragon species.
the tldr? lesser dragons: functioning members of society, greater dragons: megalomaniacs