Happiness, lasting happiness, is found in the possession of virtues required to serve a cause greater than oneself. It took me years to learn that lesson, and even now I cannot claim to heed it as faithfully as they did. But they helped to embed in my conscience a sense of self-worth that was measured by how close I approached, or how far I fell from the standard they set.

When I was in high school, I was blessed to have had an English master and football coach who also sought to teach me by example values that could give my life, and my decisions.

William B Ravenel had been a star running back and had earned a master's degree. He had served in Patton's tank corps in World War Two and survived its hard encounters with Hitler's Panzer divisions. He was, when I knew him, a lieutenant colonel in the Army Reserve and the only master at school who still served in the military. He loved English literature, especially Shakespeare, and had a very effective way of communicating his enthusiasm to us. He was a hero to his students, and we wanted to like what he liked.

I was something of a discipline problem as a boy – the problem being I didn't like discipline. I had often found myself on the weekend working off demerits I had incurred during the week for various infractions of school rules and other discourtesies. The school authorities wisely charged me with raking the leaves and doing other sundry chores in Ravenel's yard. I have no idea why he took an interest in me. At the time, I had little to recommend myself. But he did, to my great benefit.

He used the occasions to talk at length to me about everything that interested me: history, English literature, sports, girls, his war experiences and those of my father and grandfather. I began to confide in him things I rarely confided in my friends.

I told him I was bound for the Naval Academy and a career in the navy, something I wasn't particularly thrilled about at the time. He did not encourage me to refuse or accept that destiny but to consider it carefully and comprehensively, with equal attention to its sacrifices and its many compensations.

The men in my family made their careers in the military. It wasn't even discussed in terms of whether or not a son would go into the military. It was just assumed. To have considered it as a question would have been as peculiar as pondering whether I would need to shave when I grew up. But Ravenel did consider it as a question and subtly encouraged me to do so as well and to come to the conclusion that, indeed, the Navy was where I belonged. And while I cannot say that I entered the Naval Academy with any particular enthusiasm, I did not arrive there with the resentment I might have had but for his counsel.

In the years ahead, I had a pretty eventful life in the Navy and confronted decisions with consequences more fateful than one boy's career on a high school football team. To make them, I had the examples of other men, brave men who shared my circumstances, to guide me. When I was a prisoner of war, the one decision I am most proud of I made primarily because I couldn't bear the shame of their disapproval. I did not always make the right decision, then or since, but I managed many of the important ones without erring too greatly.

I can say that in every one of those serious encounters, in addition to heeding the influence of my friends and family, I imagined what Ravenel would have done and tried to do accordingly.

I am a man with public responsibilities, which I have sworn an oath to bear faithfully. My occupation requires me to make decisions that are, by that oath, intended to benefit others. They are not supposed to be made to benefit me personally or, if they do, only inasmuch as I am a citizen of the state I represent and the nation I serve. But in truth, it is asking too much of human nature to expect that exclusively personal considerations will never influence a politician.

Only the rarest of us could make such a claim, and even they would be loath to do it. But I can say that while I have made sound and not-so-sound decisions as a member of Congress – and have made the latter even when I sincerely believed them to be in the public interest – the worst decisions I have made, not just in politics but over the course of my entire life, have been those I made to seek an advantage primarily or solely for myself.

Two of the more famous or infamous occurred while I was in office. I once attended a meeting with federal regulators at the behest of a contributor to my campaign, whose interests they were investigating.

While I took no action to pressure the regulators to reconsider their investigation, had I weighed the question of honour it occasioned and the public interest more than my personal interest to render a small service to an important supporter, I would not have attended the meeting. I soon regretted that decision very much.

When I ran for president in 2000, I took a position I knew to be wrong on a controversial public issue that had a moral component because I thought it might help me win the primary in the state the issue concerned. That, too, I regretted. For in addition to the fact that it did me little political good, it caused me to be ashamed of myself, and it's a little late in life to bear that kind of burden.

In both instances, I lacked humility and an inspiration to some purpose higher than self-interest, which proved the cause of my error. But I have learned by painful experience, and I suspect I will have future occasion to relearn, that those two are the most important qualities of a good decision, and all the more so when it is a hard decision.

Hard Call: Courageous Decisions by Inspiring People, is published by Gibson Square, price £14.99. To order a copy from the Yorkshire Post Bookshop, call free on 0800 0153232 or go online at www.yorkshire postbookshop.co.uk. Postage and packing is £2.75.

John McCain is the Republican nominee for this year's US Presidential election. This is an edited extract from his new book Hard Call: Courageous Decisions by Inspiring People.