A person should make sacrifices, among others, to fulfill the obligations their deceased ancestors are no longer able to. This practice is important, for it requires a person to first learn what the obligations of their ancestors were - to understand their ancestors, and themselves. Though commonly mistaken for a worship *of* the ancestors, the difference is important to remark upon: veneration of ancestors is the beginning of fulfilling this obligation.
A human being is, biologically, more or less equal to the combination of their mother and father. They are, also, more or less, equal to a combination of two of their four grandparents. And equally two of their eight great-grandparents. And equally two of their sixteen great-great-grandparents. (and so forth). They express the limits of their biological propensities of the two halves which have combined to form them.
The capacity to attain these biological limits of potential is an expression of the individual's strength of mind, body and heart. A person may have the propensity toward alcoholism, but need not become (or remain) an alcoholic. A person may have the propensity toward any number of diseases, but may manage or even avoid them. A person may have the propensity to be quick to anger, but may learn new behaviors.
A person explores the limits of their biological potential by the help of parents, friends, teachers, colleagues, other human beings, other non-human beings, and insight. To these, a person owes a remarkable debt. Yet by attempting to repay this debt, a person comes to discover that their ancestors shared the same debt - and the victory (or defeat) of their ancestors against these very same limits.
Yet it is possible to also exceed the limits of their biological potential. And this is accomplished by merging not only the two halves two humans, but by merging the self with other selves as well. When we are separate, we are able to serve. But when we are connected through devotion, we merge. By the veneration of heroes, human or otherwise, we are able to exceed the limitations of our biology through emulation, but also by adoption: discovering new potentialities of existence, it is possible to alter the form our bodies, minds and hearts were given by biology to achieve our goals.
All disease has a cure, all injury may be healed, all behavior may be modified or changed, the limitations of understanding have no limits. Success - not failure - is our truest birthright. And just as we may have many teachers in our lives, we may become adopted by many others - human and otherwise - to gain the noble peers and lineage we lack.
Understanding the failures - and successes - of our ancestors allows us to take what was good and leave what was bad; this discernment permits us to improve ourselves as human beings. Following in their footsteps, accomplishing their sacrifices, allows us to fulfill our own destiny - and understand their hopes for us.
Therefore, support not only your parents as they supported you, but maintain your family line and traditions, become worthy of the inheritance and sacrifices your dead ancestors made for you. This is done by restraining yourself from wrongdoing, guiding yourself toward good action, training in a profession, supporting a spouse, and preparing your children to assume your duties - and those of your ancestors.
This is how this is accomplished: by attending to your teacher's lessons, desire to learn, dutifully serving your teachers, receiving instruction from them. Training in self-discipline, learn every branch of knowledge. Respect your teacher's teachers.
This is the result of the teacher-veneration: you will honor your spouse by not disrespecting them, being faithful to them, sharing authority with them, giving gifts to them. You will be well-organized, kindly disposed to your in-laws, your household workers, being faithful and loyal, dependable, looking after the household you share, skillful and diligent in all your duties. And you will respect your friends and colleagues by generosity, kind words, acting for their welfare, by being impartial and honest with them. You will protect them when they are vulnerable, protect their wealth and what they hold dear, be a refuge for them when they are afraid, do not abandon them in misfortune, honor your friendship with all their descendants.
This is the ground on which the sacrificial fire is kindled: workers, laborers, and servants should be respected: allocate work according to their aptitude, provide fair wages and food, look after their sick, share in special treats, give reasonable time off from work. Be willing to start early and finish late when necessary, take only what is given, do work well, promote your shared good reputation.
This is where the smoke from the sacrificial fire goes: you will come to treat Brahmans with respect by kindness, generosity, hospitality, and providing for material needs. You will restrain yourself from wrongdoing, guide your actions, think compassionately, do not ignore what you ought to know, clarify your knowledge, take the path to happiness.
And what is the sacrifice made upon the fire?
Simply this: yourself, to the obligations of your lineage. Find victory where your ancestors found defeat, honor their ancient alliances. Become harmless - practice Ahimsa.