This is not all the time, sometimes you gain from your investment without losing. Yes, this is true that every time you don't lose before earning or gaining profits.

We enter into market thinking we will get going and start earning right from learning something but this isn't the case and this thought or mindset only leads to the loss. We lose we learn properly and then execute.

I personally invested in stock market for long term, I didn't face this at that time. But then I thought of trading daily, I started learning and started by completing it but I failed this time because I was overconfident, lack of knowledge(Technical Analysis). If we are losing, it is due to our own fault.

Find it, sort it and the losing probability will surely decrease. I cannot say that this will never happen but this will not happen everytime.
But yes as many explained above losing or failing does teach us. It is hard but sometimes human brains/pshycology really needs it, to understand the value of anything in this World.