Because you see clothes you take on and you take off.
If I wear a chassidish garb today it doesn't make me a chassid and if I wear a Mexican pancho it doesn't make me Mexican.
But, and this is a big but, they do influence. They influence how people perceive me and even how I perceive myself.
Clothing is to the body what all our outside influences are to our soul.
Everything outside of me doesn't define who I am nor should it control my state of being. Happiness is a state of being and so is being sad. I can put it on or take it off. Just like a shirt or a skirt. BUT, and here we have a big but, it influences.
So what does this mean?
It means that just as when I dress it influences how people perceive me and how I perceive myself, so too when I surround myself with positivity or negativity it influences how I feel.
Can I build myself up so strong that nothing should matter, that nothing defines me more than I am? Wouldn't that be wonderful? Yes, but I think only at times and in theory.
So practically I do what I can to surround myself in positivity. To surround myself in beauty and kedusha (holiness). It's not what makes me, but it influences me.
And when I can't? When there is a test and it pulls me down? When there is too much going on around me that I can't change or control? I go back to remembering that this doesn't define me. It can change like a shirt or a skirt.