In arguing that one should not adopt wholesale prefabricated philosophies of life, I am not arguing that they are out of bounds entirely, or that one should not borrow from them freely. In fact, I would argue that it may be essential to study at least some religious and philosophical systems, along with the various accumulated treasures of culture, in order to compose a substantive philosophy of life.
For one thing, most people don't appear intelligent enough to do this sort of work without simply adopting the philosophies of life they inherit from their cultures and communities. They grow up with a certain religious teaching, for example, and they rely on that particular philosophy of life for as long as they live. And hopefully it serves them well. It may also be that a great many people, though not lacking in intelligence, simply don't have the time or opportunity to work on their own individual philosophies of life, and so, again, they simply adopt whatever was handed to them. It's worth saying that people who simply live according to the philosophical, moral, and practical teachings of their cultures - and who do so sincerely and well - are not deserving of scorn and contempt because they didn't compose their own unique "life's owner's manual."
Having said that, I still believe that to do this work for oneself is the highest and best path, if one can manage it. But there are at least two other reasons to lean upon the cultural, religious, and philosophical traditions of the past. First and most simply, you cannot simply invent a personal philosophy in a vacuum. You have to know what at least some other people thought about things. In fact, the more you know, the better informed you will be. So why not study the great religious texts? Why not study the great philosophers from all around the world? Why not learn about the philosophy of science and about all that science has shown us about the world, so far?
Second, though, and perhaps just as importantly, we need to lean upon received traditions because we are simply too changeable and unreliable, otherwise. Which is to say that, even though we may decide one day what we believe, or what we resolve to do and to be with a perfect sense of clarity and calm, most of us are highly inclined to forget about that a week later, or a month later, or six months later. We get distracted or swept away by our feelings or circumstances, and our prior commitment to some nascent philosophy of life gets forgotten of misplaced.
This is precisely the genius of religion. Religions know what they're doing, and that's why the great religious traditions insist that their adherents gather on a regular basis, and memorize scriptures and creeds, and engage in practices like prayer and meditation that literally rewire the human brain. The great religions know that, for most of us, our best intentions notwithstanding, we inevitably forget to live the lives we once determined to live - the truly great lives envisioned in our philosophies of life - simply because we forget to do so, or because something else captures our attention like a dog chasing a squirrel. And so we need help.
For my own part, I have spent the better part of my life reading in philosophy, theology, science, history, politics, etc. I started out as a Christian and explored every nook and cranny of Christianity I could find, but that led to explorations of Buddhism, Hinduism, Judaism, etc. One thing leads to another. Just before I left the church I started down the path of Taoism and Zen. I discovered Chuang Tzu and then T'ao Ch'ien, both of whom spoke to me, maybe more than anyone else ever had. At the same time I got into Montaigne, and his voice seemed very much like theirs. I discovered the Taoist-rooted tradition of Ch'an from which Zen sprung, and which seemed truer and better to me than Zen. And I also fell in love with Roman Stoicism, particularly as expressed by Seneca.
This path has seemed endless (endlessly wonderful to me, but maybe not always so much to the people around me), full of false starts, backtracking, stumbling, falling, ecstasy, frustration, confusion, and joy. Mostly joy. I needed to get free of the church to truly follow the path, and was immeasurably fortunate to have done so. But I would never deny that being in the church was also part of the path.
My personal philosophy of life, such as it is, is rooted very much in my personality and personal inclinations. It's rooted in the soil of Appalachia from which I came, and rooted in the life of the woods in which I grew up. If it gravitates around anything, that would be my love of science and scientific epistemology. But it goes beyond science, for the simple reason that, though I would never wish to believe anything contrary to our best science, science simply cannot speak to every need of human life.
My personal philosophy is deeply Stoic. But am I a Stoic? Probably not, at least not by anyone's textbook definition of the word. It's deeply Taoist. But am I a Taoist? Again, probably not. This philosophy of life is also deeply Western, which is to say rooted in the Judeo-Christian, Greco-Roman, Western European, and also American civilization in which I find myself. Leaving off the great religions and theologians of the ages, my personal philosophy is also no doubt shaped just as much by the music, the poetry, the movies, television series, and novels I have enjoyed, not to mention many fortunate family relationships and friendships, unexpected conversations, and life experiences.
If I'm being honest, when someone asks me what religion or philosophy I follow, I say I'm a "Mattist," which is to say that I have my own personal philosophy which probably isn't reducible in a simple way to something else. If they're interested in the details of "Mattism" - they never are, of course - I can talk about growing up in Appalachia, being formerly a Christian, the way that Taoism resonates with my mountain upbringing, the incredible practical value I've found in Stoicism, etc. Mostly, though, I suppose my philosophy of life ought to be a thing expressed in the way I live, and not in words like these. I describe this personal philosophy here, though, because I want to offer an example of what I mean to say to you, namely that you are capable of composing your own life's owner's manual in your own way and according to your own style, but I hope you will never feel constrained from diving into the wisdom and joys of the innumerable philosophies that have come before you. They are there for the taking, and I hope you will feel free to drink from those wells as deeply as you may like. I hope they will be most profitable to you.