is that I tend to get very emotionally involved with them, which often leads me to buy and sell at precisely the wrong times (in my investing experience, emotion is usually more of an enemy than a friend).

The same often holds true when people put the bulk of their portfolio in a relatively small handful of assets in an effort to "beat the market." I've witnessed my dad, who was a financial planning professor, do this throughout his investing career. While there were times when he did way better than the market averages, over the long run, he'd have been much better off by dollar cost averaging into an S&P 500 index fund (and not otherwise messing with it).

On the flip side, I don't think there's necessarily anything wrong with speculating with a relatively small percentage of one's overall portfolio as long as the risks are understood. The problem I've always had with this is that, in the grand scheme of things, I've never made that much money from doing it (all the bets I've made have been relatively small and, in any event, the good ones have largely been offset by the bad ones). So I tend to view this as something to do for entertainment purposes more than anything else.

And this isn't to say that it's not possible to consistently "beat the market" over long periods of time (Warren Buffett being a prime example). But most people who are able to do this tend to have a pretty sophisticated/disciplined investing strategy, which I have neither the time nor the talent for (and I would argue that 99% of people who try to "beat the market" don't either).