No matter what happens in life always focus on the positive.

All of us must have heard this in some or other way. It's good and the only way to adjust with your situation i guess. But to be honest I think life is more than that. I think sometimes bad things do happen to us so that we can find what's negative. Sounding crazy right. Give it a deep thought. Imagine that you prepared for your exams and you worked hard for it. But you were still not able to score up to your mark. If will be positive about it you will tell yourself its okay it happens, many people still scored less than, I should be happy with whatever I have got, etc. etc. Now think of the other scenario you will become negative about it and think I worked so hard but why the hell i was still not able to get good marks as much I expected, where did the problem occurred, where did I got less, etc. etc. So now see  you got to know your problem and eventually you will be able to solve. Being positive about is never going to help in growth and you will be happy with whatever you have got. In real life you are going to get such cases and you have to really question yourself am I really happy with whatever I am doing or should I focus on the negative find the problem and then solve it to improve my personal growth.

A fact indeed. we should always take our negative thoughts as a part of our life. It is like the yin and yang of life. You just have to find the balance between them. when we will be able to find the balance between positive and negative no one can stop us from self improvement. 

I am not saying to be negative about everything. But the things that really matter to you and are hurting you or you are not able to achieve it then yes for sure you have to look into the problem. You just not have to be negative but also find out what actually caused us too be sad, why were we not able to achieve what we wanted to achieve? questions like these and the answer to these can only be given by you and solved by you. So find the balance.