Not because everyone will let you down. Some won't.
Not because it's hard to hope. It really can be done.
Not because the world is cruel. It is, but it is also wonderful.
And not because there is nothing else worth investing in. There are many things worth your efforts.
But invest in yourself, too, because we say that the only constant in life is change. But the other constant is you. As long as you are alive, you have yourself. Invest in yourself because you will change, whether you mean to or not. And when you see these changes, you will need to have faith in order to face yourself again and again as you grow. Learn about yourself, know yourself, and grow yourself, because it is possible. And because you will need to know your constants in life, especially when everything changes.