I found an interesting blog post from Factor Investor about how much manager conviction plays a role in portfolio weightings and ultimately performance. They make the obvious observation that overweighting the "good investments" will lead to better performance. There's no real argument against the concept but the idea doesn't necessarily jibe with practical reality of the client experience or the vagaries of the stock market.
One of the things I picked in my time at Fisher Investments was that no matter what you think about anything, you could be wrong. No investor can always be right, there will be times that an investor will be wrong. One way I have framed this is to say that managing a portfolio is a series of decisions and hopefully you get more decisions correct than incorrect.
It is this idea that is the building block for my targeting 2-3% weightings for individual stocks and narrow based ETFs. If you have 30 positions in your portfolio, one of them must be the top performer and one must be the worst performer. If you had to guess which one will be the best over the next 12 months, it would be just that, a guess. I've owned Johnson & Johnson (JNJ) for clients for more than 14 years. While it is ahead of the S&P 500 in that time by a little bit, there have been plenty of calendar years where it has lagged in the index. There are times in the cycle where a more defensive stock like JNJ will outperform and other times it won't. Occasionally a stock will do the exact opposite of what you might expect based on the cycle (like a defensive stock outperforming in a raging up-year).
There are plenty of probabilities but nothing is absolutely knowable. That is why I believe in structuring a portfolio so that the wrong decisions won't damage anyone. Before you dismiss that as CYA, plenty of people reading this page are do-it-yourselfers. In a matter of speaking, I am a do-it-yourselfer. You managing your own portfolio, how much do you want to put into whatever you think will be your best performer? What is the biggest decline you've ever endured from one stock? What if you're just as wrong about what you think will now be your best performer?
I think ego is a big driver in the Factor Investor blog post and ego leads to all sorts of emotional behaviors. My personal goal for my accumulating and investing is to have enough money when I need it. If that might be one of your goals then you need to understand that you can get there without ever putting 25% into your "best idea."