Most games have some measure of chance involved. For the moment I’m mostly talking about board and card games. Games also usually have an opportunity to bring tactical or strategic thinking to bear on them–you can be “good” at them.
When you play a game where the ratio of skill and chance is shifted to one end of the spectrum, you notice. If there’s more chance than skill, you are likely to feel that it doesn’t really matter what you do. If there’s more skill than chance, you might feel like you aren’t good at the game.
I won’t say that there’s a right or wrong composition, but some things will change depending on it. A game with no chance, for example, is likely to be one that takes a significant amount of effort to feel competent playing (think chess or go). A game with no skill isn’t likely to hold the attention of adult players for long (unless money is changing hands, but that’s an entirely different kettle of fish).
For most board and card games of the sort that nerds like me buy, there is a sweet spot where the randomness keeps things from grinding to a halt due to “analysis paralysis” while enabling the players to feel more or less in control of their game play. For an excellent example of this, I suggest 7 Wonders, which I got for Christmas. It’s a game where the potential for skill is palpable, but beginners can play without being totally dominated by their more experienced peers.