Thank you for the thoughtful comment, I agree with you about relaxing.

I think that the problem with "trying hard" is not in "trying hard" itself, but in the usually accompanying side-effects such as anxiety, stress, and exhaustion. I believe that trying as hard as possible can be good in some very short-term activities (for example fighting for your life or running a 50m race), but as per your example of pursuing a career that we would really enjoy: if you focus only on that and postpone pleasures, hobbies, relaxation, friends etc. for too long, you are actually hindering you mental and physical health, and that is no good for anything.