The reason comes down to comparing.
When you compare yourself with others you set yourself up to feel badly about yourself. You over-inflate the other person’s strengths and minimise or ignore their weaknesses. While at the same time over-inflating your weaknesses and minimising your strengths.
Comparison is a losing game because you’ll always manage to find someone better than you. Instead, keep your focus on you. Measure yourself against yourself.
Sure, there’s a benefit to seeing someone much better than you perform and modeling yourself after them. They may have a certain style, skills or technique that you’d like to emulate. This is the only positive way to focus on someone else. This isn’t comparison.
Comparison usually involves a negative appraisal of yourself in relation to others. Stay away from it. Comparison is hazardous to your self-confidence and performance health.