If everything is just what it is, neither positive nor negative, then why is there about positive thinking in the teachings of yoga, especially in the lineage of Swami Sivananda?
There are two elements in the positive thinking under the teachings of yoga in the lineage of Swami Sivananda.
The first one -
The minds that are thinking in certain way that lean towards negativity even before trying to do something, such as "I can't ...," or "I won't be able to ...," then they need to change and overcome such mentality, by thinking, "I will try my best, and see what happens," without egoism. And after they have tried their best to do something without attachment, identification, craving or aversion, comparison, judgment or expectation towards the action and the fruit of action, they learn how to accept the fruit of action as it is, either "It's possible," or "It's not possible," and there's nothing wrong and it's okay, if after trying to do something, but it's not possible to do it in the present moment. While there's no pride and arrogance if it's possible of doing something with great success.
The second one -
Positive thinking is not about 'thinking positively' as what many minds would be interpreting or understanding towards 'positive thinking'.
It's not at all about "I am positive," "I am strong," "I am good enough," "I
am superhuman," "I can ...," or "I will be able to ..." in order to make
the mind feels good and confident about itself by motivating the mind
to be performing certain actions to achieve certain accomplishment in
its momentary life existence.
It's about having a healthy mental health of clarity thinking and peace of mind, by developing and having correct understanding towards the truth of selflessness and impermanence, what is the mind or the modification of the mind, how the mind works, what is going on in the modification of the mind, what is the mind behavior pattern, what is action and reaction, what is cause and effect, what is action and consequences of action, what is suffering or unhappiness, what is ignorance, what is the ego, what is egoism, what is impurity, what is restlessness, what is the root cause of suffering or unhappiness, how to be free from suffering or unhappiness, and what is the 'I' that the thinking mind thinks and believes and identifies as itself (the self-inquiry towards Who am I?), and etc, under the purified intellectual reasoning power that is reasoning everything as it is, without being influenced by any particular worldly passionate egoistic ideas, belief, values and practice.
It's about the understanding or the capability of accepting and respecting the selfless impermanent body and mind, life and the world being what it is, as it is, without egoism of attachment, identification, desire of craving and aversion, comparison, judgment and expectation. And hence, the mind is void of the suffering deriving from perceiving the ceaseless impermanent changes in all and everything in this selfless impermanent worldly life existence, that are not necessary pleasant, agreeable and desirable.
By thinking positively and being motivated to perform many kinds of actions achieving many accomplishment in life doesn't necessarily mean that the mind is or will be having correct understanding and unconditional peace, not being determined by all kinds of experiences or names and forms, being free from all kinds of suffering or unhappiness deriving from the idea of 'I', ignorance, egoism, impurities and restlessness, or being free from generating hurtful damaging behaviors, actions and reactions that hurt oneself and others and the surrounding environment.
And it's definitely not about having positive mind power to be changing and making everything to become all positive and nothing negative in the world.
Inquire the truth of everything.