Overall I get asked often how I always carry a smile (your best accessory!!) on my face and barely show any negative emotions. I think that is part of several reasons.
1) I'm a happy, harmonious person and dislike negative emotions. That is part of my character. I do always look at the bright side of life.
2) Past experiences.
3) People you surround yourself with.
4) Changing negative emotions to positive ones
So the first one is kind of in my genes, at least that is how I view it. Just like some people are negative, some have a more positive outlook.
The second reason is the experiences we have been through shape us, in my case I became a happier person, because life is not as bad as it seems. I have struggled with depression, an eating disorder and social anxiety. And it will and still is be part of me, but I do not allow it to change my mood (or try to). It is a decision you can make yourself. Am I going to let my social anxiety live my life or am I going to stand above it and do something about it. If you are not liking the situation, you are the person that can make a change.
Thirdly I'd like to address how you are influenced by the people around you, consciously and unconsciously. I also had to find that out myself, but I avoid hanging out with negative minded people. Simply because they bring you down yourself.
(I have a fellow student in my little class who is always negative, about everything. I don't really listen to what he says, goes in one ear and out another, because it used to bother me a lot)So make the change! 'Delete' those people in your social environment. You don't have to start ignoring them, but you can eat your lunch somewhere else.
At last I want to stress that it is a learning process. If you see someone who you define as beautiful, 'skinny' (i'd rather say fit) don't think that you look awful and start aiming at looking just like her. Instead think, she looks great. And leave it to that. You don't know how she feels on the insight, which is so much more important. Feeling happy and positive starts on the inside, and not fitting a size 0. Feel good on the inside, and it will show on the outside.
This doesn't mean that you will feel happier one day to another. But you have to learn to not let your negative emotions and thoughts overrule. Every positive person has negative emotions, its part of life. But it is up to you what to do with them. Go to the gym and get rid of your frustrations. Or talk or cry about it, believe me, you will feel better.
From one happy person to another