Do the people who say that really know what they're talking about? How is it bad to try too hard when you're fighting hard to get what you want, and when what you're fighting for is hard to get? It seems to run against common sense. People have lost a lot of races because they didn't try hard enough.
What possible good could come from not trying too hard.
If it comes to pushing your body to the edge of its limits, then there's nothing to be gained by not trying too hard. Bodies can be tortured into winning. Minds are more likely to be tortured into losing. Driving determination is good for the body; driving determination it's bad for the mind.
Long range goals like building a business, gaining wealth, or making a better life for yourself are matters of thought. Frenetic pushing-hard-for-the goal in these matters can easily confuse fact with assumption. Those who critically reflect on what they're doing generally do better than hard-chargers who can't concentrate on anything but the finish line.
That's the wisdom of not trying too hard.
Relaxed minds work better than agitated minds. Relaxed minds are free to notice what agitated minds run past without noticing. This is a great truism that applies to things great and small, including athletics. Relax, take a deep breath, don't get rattled . . . go!
Athletics, firemen, cops, and soldiers count on their training to get them through tough jobs. None were ever trained to rely on their passion.
This detachment from emotion is another way to understand not trying too hard. Trying too hard in these situations ends in passion overwhelming training - and reason.
There are other examples, less dramatic, though just as interesting.
Beginning instrumentalists are cautioned against practicing mistakes. That good advice isn't useful until you recognize the mistakes you've been making. Joe Walsh's advice to budding guitarists was deceptively simple, "Play what you want to hear".
If it doesn’t sound right, it isn't right.
Practicing what you've been doing wrong will only make your mistakes more consistent. Listening will achieve more than doing, no matter how hard you do the doing.
Pushing hard hardly matters when the goal isn't worth getting to. Unexamined goals are often not worth getting to. Effort and commitment will accomplish nothing unless guided by reason and analysis.
Trying too hard means trying too hard to do the wrong thing.
Sisyphus was never able to push his rock to the top of the mountain. The rock crashed back to the bottom every time he got it almost to the top. He was cursed by the gods to keep trying anyway.
I wonder if Sisyphus ever worried he might be tryin' too hard.