Being on a forum like this, I'm sure the majority of us have heard the cliched advice, "If you think you're going to fail...it becomes a self-fulfilling prophesy..." Then they tell you to adopt a more positive outlook on life and that will somehow bring positive things into your life...
This is all well and good, but the reason this advice probably doesn't dynamically change anyone's thought process and behavior is because it usually isn't explained in a logically sound argument, and, instead makes you feel like you have to rely on "good things just coming to you if you believe it." For those who need logically convincing arguments, this is for you.
What a human being is constantly searching for is truth. Am I really here? Is the sky really blue? Most of this search is assumed at a very early age, which is generally good but has a few quirks. If someone believes strongly in something then they will very likely continue to believe in it, unless something comes in and shakes up their reality in a very dynamic way. To use an example, let's say a kid gets picked on by his father for not being big, tall, and strong enough compared to the other boys his age. In all likelihood, if he believed his father that he was very weak compared to other boys he may hate his father for picking on him about it, but he will still internalize those beliefs about himself. He will still view himself as weak and small and deficient in this area. Of course, the kid could adopt a mindset that, screw you dad, I'm fast and agile and powerful and can beat any of those kids, but, even in this case, if he internalizes that his dad likes the other boys better he will adopt the mindset that other boys are more likable because they have those attributes that he does not have.
In any event, the boy will likely continue to believe he is weak and small as he matures into adulthood. He may try to find ways to compensate for this percieved flaw, but he will always believe he has it. He will always believe he has it, and even when he attempts to to say, well, it's ok that I'm weak and small, people don't care about that...it won't really work because, again, he believes his reality, his belief system, is that he is weak and small and people view him negativily because of it. The reason he can't even convince himself is because, to go back to the original thesiss, a human being is always searching for truth. Because it has entered his understanding of the world, the boy believes it is truth similar to how he believes the sky is blue. While the latter is maybe a slightly extreme example, to break away from a belief that helps him understand the world will make life very confusing and also his mind will resist any attempts to break away the belief, because the belief is held so strongly it appears to be fact.
So, how does this affect our interaction with other people? It tells us how we should judge ourselves in a given social situation. Again, it isn't bad to assume things, we have to have some mechanism of assumption or otherwise be overloaded with new information all the time. However, to use our example again, the boy will most likely assume the things that his father believed about him were true and, if the believe is strong enough, he will impose that reality onto whomever he is interacting with. In other words, if he is dating a girl, for instance, and she is friendly with a very muscular athelete, he, of course, will be jealous, but worse he will impose that reality onto the girl he is dating and she will believe that the boy is right...that he is weak, small, and inferior to the athlete. And, to use an example that's perhaps less rude on the girls part, let's just say a guy meets a girl at a party and he believes that he always bores girls because he can never think of anything to say. Well, she may come in being excited to meet this attractive, new guy, but then the strength of his reality that he is boring convinces her that he is boring because he will only act in ways that he believes are boring because that is what his mind is telling him is what he is. He can't act charismatically when he feels boring because it doesn't feel true, and if he does manage to it is either a transparent effort or alcohol-induced effort. Conversely, if his father always believed the boy was better than the other boys, the boy will most likely believe he's better than the other boys even if he is small and weak and the athlete won't be a threat, and the girl will again, if it's strong, fall under the boy's strong belief that he is better than the athlete because she, too, is searching for the truth of things.
So, to cap it off, self-fulling prophesies are a real thing, but it's not as easy as read positive phrases all day, you really need to challenge your mind with all new beliefs about yourself and then the behavior will follow. Otherwise, you will be fighting a very uphill battle, where you are trying to accomplish something you don't actually feel capable of. Your mind is a tool like your arms or your legs and can be manipulated to adopt new beliefs and you can even do it today. The reason you won't isn't because you need practice. The reason you won't is because you'll be scared as hell because there is no foundation for this new belief to help you believe it's true and you feel like something bad might happen if you believe it. Nothing will.