A young Warren Buffett had 75% of his allocation in GEICO.

If you're certain something is going to go up in the long term, it makes sense to buy as much of it as you can. You must be certain, and you'd better be right about being certain.

A Buffett-favored aphorism: "Too much of a good thing is wonderful."

If your goal is substantially different performance from a market average, a concentrated position is the ticket. But, unless you're really good, the best you'll do, on average, is average, in which case you'd do better by buying an index.