Why are we trying so hard to be something

When we do yoga, why ideas of being something, fitter, more muscular, a good person or being a yoga teacher one day comes to our mind? As we would need to gain something, appreciation or admiration with doing what we are doing.
Why can't we just be nothing. Just be what we really are at the moment.

When I'm in yoga class and notice that I'm trying too much, I ask myself what and why am I trying? Please the teacher or show something for the others?
One day when I was doing asanas, my teacher came to correct me and said that even the force that I have when doing asanas comes outside, it's not mine. Don't try too hard.
I'm nothing. It is somehow a relief, as releasing yourself. There is something bigger than I.
That helps me to put my ego down and all that shaking with it. Just be and do what you are doing.

Another thing is that as much as I admire my teacher, doing yoga regular does not mean that I'm doing it for a goal to be yoga teacher one day. That is very general thinking among some people. As to do something I have to have some understandable goal.
I do astanga yoga for the joy of knowing myself more, it helps me work my emotions and control my very fluctuating mind. It helps me to forget and solve what is stressful and pressing in my life and to get beyond those feelings.
I love feeling strong physically in my body, but even more in my mind. I like the repeating way of Asanas in traditional Ashtanga yoga. Moments when you don't have anything else in your mind than breathing. I'm the breathing. We come one, the breathing and the movement. The feeling when the rhythm of the asanas takes you into it's journey. And then, you just flow by the stream.
I think yoga and especially Ashtanga applies commitment and flexibility at the same time. To listen really below the surface and to know when it's time to take more easy.

I think yoga is a life long journey inside of you. You continue or you don't.
Do yoga for yourself without thinking what it will help you to be or gain someday. Don't nourish your ego with ideas of others thinking what you are when doing something, just do it.
Everything will come one day or it will not. As the master Sri K.Pattabhi Jois once said "Do your practice and all is coming".