Because you need to be
certain it's what you want to do. You need to
search deep within yourself,
experiment to feel what really makes you
happy and
content, and find a way to
discard all those
parts of you that are
merely there because you're trying to be what
other people want you to be.
The last one is the key. Most of us are trying to be what others want and expect us to be. We live by the rules that are set out for us, by parents and society, enforced by various processes that make it clear what we want should align with what other people want us to want.
It's a tough fight to shed all that baggage, to stop doing things to fit expectations, and to find who you really are. And it's made even tougher due to self-confidence and/or lack thereof, and acceptance. This is my toughest area in trying to find myself, that my self-confidence, my self-esteem, is really low, and I need that acceptance to really feel good about who I am. I'm fighting to tell myself that who I am is good enough regardless of what other people think.
Being yourself is one of the toughest things to do, and many people just can't do it.